As part of my business I have cause to replace stickers quite often...so here's just a couple of handy hints and tips, to hopefully get it right first time!
Removing them.....
1) Get them warm - leaving out in strong sunshine works well and much persistance with fingernails and gentle persuasion!
2) Once you get the sticker part off and are just left with the glue - use tar remover (available from Halfords etc - Autoglym very good) or you can get the professional stuff from Autosmart (called Tardis) which does tar, glue etc.
3) Wash the area you want to sticker with mild detergent, allow to dry, then T-cut (...fastest way to get T-cut off paintwork is to use a cheap liquid wax over the top, then polish both off (..stops it going dry and cakey))
Application......
1) Thoroughly wet the bodywork with warm water and as you peel the sticker from the backing, spray this with water also (use one of those mister things used for plants)..this allows the sticker to be moved without grabbing the paintwork (...avoiding much peeling on and off!)
2) Once your happy with where you want it - Hold one end securely and squeeze the excess water out, whilst flattening the sticker, with an old wiper blade or the squeegie part of an ice scraper.
3) Leave to dry - away from dogs, cats and over zealous admiration of ya handywork
Hope this helps! :D