Set your hair dryer to its hottest setting and balst the sticker with hot air for 30 secs or so. you will find that the glue very quickly starts to soften/melt, allowing the sticker to peel off the sole cleanly and easily, leaving no residue!!
This can technique can also be used to remove sticky labels from tags and other items when needing to give them as a presant!
Goo Gone is the easiest product for this, and a bottle lasts for years. Also great for use on glass, wood or anything else with that annoying sticker residue.
http://www.axminster.co.uk/product.asp?pf_id=376969&src=froogle
Sticky Stuff Remover is also a brill way of removing this, pretty much the same stuff as that Goo Gone by the sounds of it but you can get this from the Bettaware catalogue. This stuff is also really good for removing chewing gum from clothes.
Avon Skin-so-soft. Spray on the sticker and wipe off with a paper towell. A little scrubbing is required. If it is really stuck apply SSS and let set for 5-10 min (repeat if necessary) and scrub. Then wash the item with Dawn or something like that to remove all the oily substance.