If you do any sort of DIY, mechanical, construction, carpentry, or even crafting, you're going to work with screws, and sooner or later, the heads will get stripped. It's annoying, but it's not the end of your project. Here's a couple ideas to salvage your work.
1). Use a manual screwdriver. You can control the speed and friction alot better with a manual screwdriver rather than a powerdrill or driver.
2). Use a screw extractor. These handy tools have tapered threads that are set up counter-clockwise fromtraditonal screw threads. The threads dig into the screw, thus turning it backwards (lefty loosey) and eventually removing it.
3). Use a punch. You can also use a metal punch or nail punch (sink) to force the screw in the left direction without relying on the screw heads. Here's a video detailing the process.
4). Drill it out. As a last resort, you can remove the screw head completely by drilling away the metal with progressively larger bits until only the shaft is left, then removing the top piece through the hole, and yanking out the threaded shaft with a pair of pliers.
Images from w6rec.com; qwikfast.co.uk; m-99.co.uk