Just another thought on choosing affiliate programs.  While more and more businesses are launching affiliate programs, surely some of these will be happy to benefit from your promotional efforts while rewarding you very little, or nothing at all.

Certainly, some companies will pay affiliates whether their customers pay online or off.  But I suspect that many will not pay the person who sent them the customer if they can get away with it.  An offline payment would seem to make this outcome more likely.  So, businesses that are known for having a high percentage of online transactions -- perhaps with discounts for paying this way -- would be most lucrative for marketers. 

Also, in choosing affiliate programs I'd say it's worth finding big businesses that are well-managed and scrutinised.  So, if they say they'll pay their affiliates so much, you can be pretty sure they will.

For these two reasons I think that big online travel agencies may well be a good place to look.  There's also the fact that with the economy booming (at least in Australia) more and more people are travelling interstate and overseas, so the percentage of leads that convert is higher.