My advice is seriously DON'T BOTHER !
It's only going to reduce the overall quality of your iPhone. Having owned every previous incarnation and always protecting with a screen protector, i decided on my latest 4S, to not bother, since the glass is apparently scratch resistant.. Admitedly I was nervous at the time, thinking would this be the costliest decision I've ever made, but since October, my iPhone has only been protected by a bumper case, and I can safely say its been the BEST decision I've made.. I get to appreciate the true optical quality of the glass front and back, after, what's it been now, 6months of use.. There is not so much as even a hairline scratch on either side of my glass. The phone could still pass as brand new.
The screen protectors I've used in the past, on my 4, 3G-S, 3G all had screen protectors and you could always tell. The softer plastic would always get harline scratches and you'd have to replace every few months to keep the glass optimal. I've used a bumper since the 4, so had to run with a protector front and back, but on my 4S, nothing front or back and the phone goes everywhere with me, and not so much as even a light scratch. Obviously I've taken precautions such as never just throwing my phone anywhere i want to, but with the screen protectors being so delicate and easily scatchable, you had to take those precautions anyway.
As above, don't bother if you have a 4 or above.
oh and edit, i also let my 6 yr old play with my phone on a regular basis.. If its stood the test of time in the hands of a six yr old, then no adult should have a problem.