Do some research first before you join an affiliate program. Find out a bit about the programs that you might consider joining. Do they cost you anything to join? Most affiliate programs are free to join, after all as well as making money for yourself you will be making money for the owner, So why join those that charge you for joining.
When do they issue the commission checks? Each program may be different. Some of them will issue their checks once a month, or once every two weeks others every quarter, etc. Many affiliate programs will have a minimum payment amount that the affiliate must meet in order for their payments to be issued.
If you are promoting affiliate products through clickbank for example you can be sure of accurate tracking of you clicks/visits and regular payment every 2 weeks
Find out how referrals are tracked and for how long do they remain in the system? You need to be able to trust the program enough to track the traffic you send so that you get paid your commission as this is the only way you will get credit for that sale. The period of time that customers stay in the system is important. Some visitors will not buy straight away but may return later to make the purchase and you need to make sure you will still get credit for the sale if it is done days or even weeks after the initial visit.
Are affiliate stats available? Your choice of affiliate program should be capable of offering you detailed stats. Ideally you need to be able to track how many visitors, impressions and sales you are achieving. A hit is one click on the banner or text links.
Some affiliate programs may also pay for the hits and impressions besides the commissions on sales or pay a CPA (cost per action) for example you get paid when they enter their details.
Is the online retailer a reputable source? Being able to check how many sales they are making through affiliates is useful in determining if the offer is converting and is very profitable for affiliates.
Is it a one tier or two tier program? A single tier program pays you only for the leads or sales you yourself generate. A two tier program pays you this but also pays you a commission on the on the sales generated by any other affiliates you sponsor into the program. Some programs may pay for each new affiliate you sponsor.
What tools do they provide you with? Some programs will provide you with banner ads, articles, text ads, links, email copy etc, a good affiliate program will provide you with plenty of marketing materials.
Finally very important is the amount of commission being paid? i.e 50%, or 75% (or in some cases even 100%!) If you find a CPA program that also pays for impressions, or actions the amount paid is much less and usually fixed price per lead for example not a percentage of sale.
These are just some of the important questions that you need answering before you join any affiliate program.