Go to Riverwood and get Faendal as your follower. Ask him to train you in archery and pay him the gold. After gaining a level talk to him and ask to trade with him. Take the gold back out of his inventory and then ask him to train another level. Rinse and repeat infinitely. This also works with any of the Companion followers who are also trainers in Whiterun. (Aela, Athis, Farkas, Njada and Vilkas).