I cleaned my bumper with isopropyl alcohol. 3M also has an adhesive promoter that goes with their tape. Comes in little packets like a hand wipe. I bought a whole box a couple of years ago as I figured it would come in handy. That and two strips of tape, I don't think I'll have any problems with it coming off.
I used 3m indoor/outdoor double sided tape. You can get it at home depot when you buy the lip. I just cleaned the lip and the bumper with rubbing alcohol, and I've had mine on for 7months. No problems.
30K Car and 10 bucks worth of garage sealer, priceless. I'll be glad when someone actually makes an abs or cf lip for this car (other than the weirdo looking Autoxe splitter).