I always end up ripping these things off out of complete frustration in trying to get them on perfectly. If it isn't air bubbles, it's a piece of dust or something.
On a side note, I HIGHLY recommend getting some of this stuff from Klear Screen. It cleans really well and makes the touchscreen feel like glass. I've been using it now for several years. Apple resells the same thing under their own brand name as iKlear but it's more expensive through them.
http://www.klearscreen.com/