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. If you're a referring site, plan your revolt. 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. Now watch what happens to your bottom line. If you're a referring site, ask for more.