Pick your spot
Gripz are basically stickers - once you put them on, they will stay put unless you peel them off. So first you want to have an idea of which sticker you are putting where. I've listed a few placements below that work for me - but ultimately it is what feels right for you. They come off pretty easily at first - so feel free to experiment.
Clean the surface
So you want a clean surface to apply them too. Oil from your hands is the most common issue - - just use a cotton ball and a bit of rubbing alcohol to clean off the surface. Let it dry and apply the
Stick em on
Peel off the adhesive backing, and press them in place. You want to make sure they are pressed against the plastic and give them a few minutes to bond.
Sample Placement
Keyboard: Here is my left hand warcraft setup. This lets me consitently hit "stretch" keys like "1" "H" "B" without even thinking about it. Some of the keys I use the triangle shape, because I tend to hit them in a specific place, and it adds an additional layer of tactile difference between the keys.
Apple iPhone: That is my phone in the left column - I do two strips down the side, and the logo on the back. I also clip a little bit off the left side Gripz, to make room for the volume controls. NOTE: I was showing off my gripz the other day and "slid" it across the top of a friends office desk - it "gripped" alright, and scraped the S out of it. So yeah, don't do that.
Xbox 360: Logos on the top where my palm rests, and two large triangles on the underside where my fingers rest. Don't put them too far forward... 1) They won't hold onto the curved surface. If yours start popping off in this location, slide them back a bit. 2) Your finger tips actually wrap around pretty far back - much further than I original thought.
Test it for yourself. Hold your controller lightly in your hand - where are your contact points? That is where you want to place your Gripz!
Removing and Replacing Gripz
I swap out my Gripz at different rates. My phone goes through them the quickest - I change them every 3-4 months. Why? Although they might seem really rough at first, gripz wear down - and I use the phone throughout the day. I'm pretty sure the extra heat that my iphone kicks off breaks down the glue. When yours start to slide around, or just look dingy - replacement is pretty easy.
Just peel them off, wipe down the surface with a bit of rubbing alcohol and apply a new set.