Make sure to look into referral programs on the various sites. Many of them will offer rewards for users that you refer to the site either as photographers or stock image purchasers. You can send your referral link to interested people by email or include the link on your personal website. Shutterstock and Dreamstime are particularly good for rewarding successful referrals.

Example: Shutterstock
Each time a photographer that you referred makes a sale, you collect $0.03 and when you refer a buyer, you get 20% of the sale (which could be $30-$50 or more).

Photo by Pierre Amerlynck.