In the long run, if affiliate programs are going to work, they must be more equitable. If you're a transaction site, prepare yourself for the day when there will be a revolt from your referring sites. Affiliate program hosts, you must offer more to your referrers. Give them enough to get their attention. The more you give, the more they'll refer.
Financial incentives are always the ultimate motivator.
In fact, instead of a percentage, I suggest a generous flat referral fee. Now look at the benefit to the referring site. Like three times. If you're a referring site, ask for more. Remember, this is a no-risk proposition for the Web store.