- Whether you use a rubber stamp or just write the URL on the bill, don't obscure the bill to the point that it cannot be used. Writing on bills is legal, making them unusable is not. (Some vending machines won't take bills if you make the mark too large - this does not mean the bill is unusable. Just give it to a real person.)
- Spend in locations that are likely to have good turnaround. Convenience stores, tollbooths, and fast food restaurants are good places; vending machines are not.
- Only enter bills that you have in front of you. If you want to mark a bill but you're not in front of the computer, don't enter it later - this could result in a false "reverse" hit. Mark the bill and send it out - perhaps someone else will start Georging someday. (If you are in front of a computer and have no way to mark a bill, go ahead and enter it - but if it leaves your hands unmarked, don't expect it to hit very soon!)
- Give cashiers larger bills for smaller purchases. They won't like this very much, but you will get more bills in change, which can then be marked.
- Be patient.
My source for this was personal experience but additional information may be found at wheresgeorge.com (official site) and wheresgeorge.net (fan site).