If you're doing a great job for the customer they will tell others about you, you don't have to offer an incentive for that.
If you're a designer, you should be teamed up with a print shop and hosting company. If you're in hosting you should be teamed with marketing, programmers and designers.
Offline media works (at least for us). Whether it's a flyer or business card in Dublin Ireland or a business card at a local resturant in Omaha Nebraksa, meeting someone at a bar in Las Vegas Nevada, or playing golf in Palm Springs California - hand your busienss card to EVERY person you meet. If you're not actively promoting your business, then your business is not growing. EVERYONE takes a business card from your hand (even if they think they don't want it). It costs less than $30 for a couple thousand cards - well worth the money for the potential return.
Go the next step - get a t-shirt print screener and print up T-Shirts. Costs are reduced to less than $4.00 per shirt if you make them yourself. Hand them out, send them to clients - it's marketing. You must advertise! And when all else fails, go back to the old marketing saying "you have to spend money to make money".
If you're not familiar with marketing techniques, hire a marketing person or a marketing consulting session with people who do this all day every day. I came from a marketing background (hotels and a waterpark), so launching into my own business was a fairly easy step as the same markeing principals carry over.