I use the static cling white background and static cling numbers from GoChecker.com.
The static cling has to have any detailer spray and wax removed or they won't stick well. I use a wax remover solution to remove it. Once car is back home the decals pull off and can be reused down the road. Just put some wax back on when done....never had them pull off at speed. I ordered with full set of numbers so I can use different numbers if necessary if I can't get the same number.
The numbers here in Michigan are assigned permanently for those that compete for points every year and run all the SCCA events for points. Otherwise you hope to get the number you first select but someone may beat you to that number. To avoid this, select a higher number and you hopefully won't have two people asking for that number. My letters on the white background are static also so just pull them off and save for next time. This is the way to go when your car is primarily a street driven car and you don't want to leave the numbers on all the time. The white background looks good but is an extra step. A simpler way is to just get contrasting color to your paint numbers and don't use the white background circle or oval.