I have been asked many times over the years, How do I choose the right sponcer. There really isnt a wrong way, but there is some clues to help you choose sponcers that are going to help you in the long run. First reputable sponcers dont spam you, they can loose there pipeline if they do. Second off they dont do it. If I get spammed from a sponcer, the email is dumped.. It helps if the sponcer has been around along time, or if it a new sponcer that you have seen the person post on the boards. Not just one post, but many..
The first thing you should do when considering a new sponcer is read there tos, or fact, This is there little key to how they operate if they dont have a tos, you really have no contract with them either way, and there not worth useing.
So once you have read there tos and you agree to it, save it in a folder on your hard drive. second thing to do is email them, any reputable sponcer is going to reply to your email in 24 hours if they dont, or the email bounces trash there tos on your drive and go elsewhere. Make sure you email them on a weekday, cause weekends dont count in the 24 hour plan.
When I email them I norm ask them for there phone number so that I can call and talk to there staff, If they refuse, there trashed. If my stats are screwed I want to talk to a live person, and email dont get it.
I go to netsol and I check where there main offices is, if they have a yahoo or free email service running there mail, there dumped. Any reputable sponcer is going to have a real email listed on netsol.
Sometimes you may have to go to another site but it is norm listed on netsol where this other site is to look them up. If a sponcer you are chooseing to use for a partnership and you get % of the signup plus reacurring, then you need to see what the inside of the site looks like to make that decision dont just asume sence a sponcer gives you this option that the site is updated regularly, this is very important when you decide to do a partnership because if the same crap month after month your not going to be able to keep a % client base and make money on a partnership. Look very closely at who proccess there credit cards, If its some wacked out company in russia you tend to want to shy away from them.
The next thing is very important, do they have tech support to where they have someone that knows the biz and can help you with your pages, if they don't and they don't know how to sell there own product, how they going to help you.
Most reliable sponcers have someone on staff that can help you. Alot of newbie webmasters dont know this, but there is always someone around a reputable sponcer that knows whats going on and can help you. The next thing you should do is check out the sites they want you to sell, if it looks like shit you are not going to be able to sell it. It dont matter if its free to join or cost 80 bucks if the sites are not sellable your not going to be able to sell them, period.
I used to look a sponcers banners, but every newbie or oldbee should know, that makeing custom banners and txt links is your responcability, so this isnt as important as it used to be.
There is also something else you can do, do a lookup on alexa
http://www.alexa.com/ see what there stats look like, how much traffic there getting, if the sites your promoting is getting to much traffic or not enough traffic, then you kind of need to make a dicision if you wont to promote them.
There is many factors to every sponcer and every situation, not every sponcer is good for every webmaster, your the one that has to judge that. Some webmasters are better at selling once sponcer while others are better at selling another, this is a trial and error thing, you should try many sponcers and find the sites that are easyest for you to make a sale