Right, you're not James Bond. To make GPS work for you, get a rough understanding of how the system works and a more detailed understanding of how your receiver is operated. Two hours of concentrated reading of the manual and playing with the receiver will give you a pretty good start.
GPS is a very robust system: Whenever you get lost, blame it on yourself.
What GPS is not
A GPS receiver is not a magnetic compass. Unless a magnetic compass has been integrated into the receiver, it cannot tell you where is North. However, it can tell you which direction you are MOVING, and while you are moving, it will know where North is. If you are NOT MOVING, it won't know / will indicate wrongly.
A basic GPS receiver will not tell you where you must turn left or right. It can tell you in which direction and how far away your destination is, but no promise that you can move along a straight line from you current position to your destination. You may have to obey traffic rules, go around mountains or look for bridges. You might have to go back in order to get on a trail that will eventually reach your destination.
A GPS receiver is not a rock. It can lose it's data or break down completely. Not too often, but it happens.
Getting to your destination
You have destination waypoint & track stored in the GPS:
Follow the track until you arrive. For this, view the map / track page. A scale of around 3km is usually best.
You have destination waypoint & several waypoints along the way in the GPS:
Go to the first waypoint, then the second, the third, .. until you reach the destination. Whenever the road deviates too far from your intended course without that there is an obvious reason (mountain, river), consider to turn left / right.
You have only the destination waypoint in the GPS:
For longer distances, consult a map and identify a fast road that can take you near your destination. If you don't do that, you may put up with a small road where 10km to the west runs the Autobahn in parallel.
For shorter distances, take a road that roughly goes into the direction of your destination. Whenever the road deviates too far from your intended course without that there is an obvious reason (mountain, river), consider to turn left / right.
You have nothing but a map and a GPS with clear memory:
Mark on the map your start position and some key waypoints along the way. From the map, find out how far your first key waypoint is away and which direction it is. Use this info to create a new waypoint, relative to your position. Go to that new waypoint but consider it won't be accurate. As you get near, look at signs and landmarks. Once you reach this waypoint, delete your predicted waypoint and store the actual location on the GPS. Predict the next waypoint and so forth...
You have nothing but a GPS with clear memory. You know how to spell or pronounce your destination:
Check for roadsigns and ask people. Store landmarks on the GPS. Keep the track plotter running and avoid going round in circles. You are now mapping new terrain.
I want to go N-NE ...
..but there's no road:
There might not be a road, you have to go some distance into another direction
..and there's this dirtroad which seems to go exactly there:
There might be a better parallel road, go some distance along a good road to find out. Dirtroads are a lot slower and bad for the vehicle.
..but then I have to cross a stream. The road continues on the other side:
A major detour could be faster and safer than crossing a stream where your vehicle might get stuck.
..but there's a huge mountain in that direction:
You want to be behind that mountain. Ignore the GPS and find a way around the mountain, first.
..which means I go along this zig-zag river:
Often it pays to drive away from a river, as the zig-zag road along the river is much longer than a possibly existing more direct parallel road.
..but suddenly the road changes direction, so should I find a turnoff ?
If you are inside a village, town, city, follow traffic signs or continue on the mainroad. Ignore the GPS if your destination is far away.
If the road is following along a river, go straight until it either comes back to the right direction or a bridge offers you passage across.
If there's no reason that the road would have to turn but nevertheless it does, then take a turnoff.
I received Latitude / Longitude of a spot but when I went there, it was kilometers off !
You got the Map Datum wrong.
Less common applications of GPS
Your vehicle breaks down, you have to leave it and want to come back later.
You want to hide some of your equipment in the forest and pick it up later on the way back. With GPS, you find the hiding place back.
You found a freshwater source, camp ground, food. You expect more to come along the road that you travel. If not, you may want to return to this place. GPS can tell you how far you are away so you can decide if it's worth it.
Where am I ?
If completely lost (let's say you parachuted off an airliner), you will still know exactly where you are but this won't help you. You want to know what is around you and where you can go. Check the "nearest waypoints" on the GPS.
If you are far from any previously-stored waypoints, run the track plotter of the GPS and move (not in circles). Mark major landmarks as you pass them. After some km, you may be able to find a match between the track that you plotted and a track you see on the map. If you don't have a map, the situation looks dim. Ask someone where you are, a taxidriver would be the right person.
Should there be no people at all, try to find a human settlement and use the GPS to store any useful spot you come across (freshwater, shelter,...). For the rest, apply common low-tech sense: Human settlements tend to be along roads and rivers, not so much on mountains. Where dogs dwell, there are people, an abandoned house has no dogs. Fields that have recently been worked on suggest that people are nearby. Looking around from atop a hill at night, you may see lights that indicate the presence of humans. Good luck.
Avoiding / managing hardware breakdowns
Most GPS receivers are designed to withstand hostile environment, nevertheless they will suffer and eventually break down. It's a matter of time. Avoid exposure to extreme heat, cold, shock, water, to make them last longer. Retire the equipment after a given time or if unexpected behaviour occurrs (you could sell an old unit 2nd hand and buy a newer model, so your loss would be minimized).
If you rely on GPS, it's not a bad idea to have one or two backup units handy. Data should be syncronized between all receivers on a regular basis and available batteries must match the backup units. In any case, carry a magnetic compass as a last resort.
A GPS with a broken display may still retain data. Operate the set blindly, to initiate data transfer to a working unit or a PC.
..deserve special attention. If you're in the middle of nowhere, got there by GPS and need it for your return, running out of batteries is bad.
Carry enough backup batteries. You can reduce the amount, if your other electric equipment (flashlight, radio, pump, ...) uses the same type of batteries.
If batteries are subjected to shock, they can internally break, reducing their useful capacity more or less. Avoid storing batteries in places where they can hit something. Store not all batteries in the same spot.
If a set of 4 cells goes flat far earlier than expected, this is usually due to one out of the 4 cells. Try to identify the one and discard it. Keep the other 3 half-depleted cells for later use.
Common AA-cells discharge within a day, once they get wet. You may not be able to prevent those inside the GPS from getting wet, but be sure your other supplies remain dry. Rechargeable batteries are an advantage: If discharged, simply recharge them. Assuming you have means to do this.
In most civilized countries good-quality alkaline cells are standard. Different in developing countries. In Thailand, a very popular Duracell-imitation is being sold in the markets. These imitations give less than 1h runtime in a GPS12, while the originals last 20h. Imagine what huge number of spares you'd have to carry ! If batteries are selling surprisingly cheap or weigh surprisingly little, something is wrong. Buy from reputated (foreign-managed) outlets. Always buy foreign brands. They may be produced locally, but according to international quality standards. If you have a choice, don't buy Duracell, which is the brand with the highest reputation = most likely to be cloned. If in doubt about the quality, consume one set in a high-power application (strong flashlight) to see if they meet your expectations. Test not only one cell, test at least 4.
If quality of available batteries is too bad, use rechargeables. In severe cases, you may have to use a lead-acid on a wrist strap (available all over the 3rd world for motorcycles or use with headlamps), wire this up with the GPS. Be sure about voltage and polarity. Careful when charging the battery, can be too high of a voltage for the GPS (motorcycle !)