Bicycle Mechanics - Constantly bending axles!

About 2 years ago i bought a GT Agressor 3.0 from Halfords. All was going well until i buckled my back wheel. I then bought a new wheel complete from halfords (26") and that was also ok until the axle snapped, so again i bought another wheel. Since then i seem to be bending axle after axle. I have tried just changing the axle its self as well as replacing the entire wheel. I deffinatly don't abuse the wheel and the last one bent today just cycling up a country lane. Could it be the frame or the gears?
Cheers

I'm not familiar with Halfords. Is it a bike shop or a department store?

The Aggressor is an entry level bike that comes with coresponding entry level parts, including the wheels. I am guessing that you are not a featherweight and/or you have been riding the bike like you would expect a mountain bike should be ridden. I am also guessing that when you replaced the axle and rear wheel you bought less expensive pieces that were a quality match for the original stuff.

You are going to have to buy a better quality wheel. You may be able to buy a stonger axle, but it may just be better to buck up and get a higher end wheel. If you try to replace the axle again, make sure you get a good chrome-moly axle. You may have to get new cones and bearings to make it work with the rest of your wheel. If you get e new wheel, make sure it is a good quality wheel, like a Shimano LX or better hub and correspondingly better quality rim, etc. The bike lists for about $300.00. If you want to get a wheel that will last, you will have to spend over $100.00 for the wheel. Don't bother with a $50 replacement wheel, it won't last unless you are under 120lbs, only ride on the street, and don't do jumps higher than curbs.

About 2 years ago i bought a GT Agressor 3.0 from Halfords. All was going well until i buckled my back wheel. I then bought a new wheel complete from halfords (26") and that was also ok until the axle snapped, so again i bought another wheel. Since then i seem to be bending axle after axle. I have tried just changing the axle its self as well as replacing the entire wheel. I deffinatly don't abuse the wheel and the last one bent today just cycling up a country lane. Could it be the frame or the gears?
Cheers

It could indeed be the frame. Better than average bike shops have a gauge (looks like two mushrooms attached to metal handles) to check to see if the dropouts are parallel. If they aren't, tightening the axle in the dropouts will tend to make it bend. The same gauge, by the way, provides a nice lever for putting them right.

If you bought a cheap mountain bike at a department store, it probably has a freewheel rear hub, instead of a cassette hub. Sheldon Brown has some pics and explanation here (http://www.sheldonbrown.com/freewheels.html). Basically, a freewheel hub leaves a lot more axle exposed between the bearings and the frame, and the design is much more susceptible to bending or snapping than a freehub, which has the bearings on the outer edge of the gears.
You can buy high-quality track axles made of cro-moly steel, that are less likely to bend, but you'd probably be better off buying a rear wheel with a cassette hub.