Ad-Exchange or Referall
What would you prefere, others to see your banner or money? If you want money then you normally get money per sale or click-through, (like a percentage or comminsion) or if a Guestbook or Feedback form is filled in.
If you want an Ad-Exchange then this is done on a ratio basis as described above

The problem with referall programs
Most referal programs have a minimum pay out fee, and this is what to look for when chosing your refferal partner. How much per clickthrough? some only offer $0.02 dollas per click through some upto $0.46, both these examples are from clicktrade. But they may well have a minimum of $20. Turbocom.net does not have a minimum but they pay a (nice) commision not per clickthrough I think they are the most advanced and if you are a member of Multi-Exchange or Traffic X you may well have there button on your site anyway.

Prioritys; High ratio or Extras/perks?
If you chose an ad-exchange program then the main Question is what do you think is more important, a high ratio or better stats? But remember you can get stats free from somone like EXTREME TRAKING but if you want the sort of traking you get from LE, being full site/ multipage results they do charge (even if it is quite cheap). N.B you may be able to get good stats from your ISP (Internet Service Provider)

What size: Banner, button, board or link?
Though meny offer both this is still important, especially if you have a really nice graphic of a specif size. The common Banner size is 468*60 and buttons are about 31*88

Now you have got your preferances its time to choise your network.
The specs for meny Exchanges are on one page. The quick refference below for more information on the biggest best and most popular ones

ClickThru Network	This is a very good deal, 1:1 click thurs. You can get credits from going to other members sites (by clicking on the click-thru logo) or by sending other people to a members site with a banner or the ClickThru Network Random Link
Multi-exchange	Best for ratio, 5:4 instead of 2:1 which makes a big difference. Plus you can have more than one Button/banner, and you get 300 free hits when you join. The code they give you can also be changed on each page which sergests that they will soon be doing the same stats as LE. If they do they will be by far the best.
Link Exchange Duluk	Best for stats, you put a slightly different code on each page so that you can see which page has the most Hits. You have a list of your pages and the the amount of hits they recieved
Traffic x	Best for Garrented traffic, rather than how meny times an advert is seen this is done by how meny times the advert is clicked on. With a 2:1 ratio, this has worked out very well for me as lots of people have clicked on buttons on my site which also surgests that the sites on this network (or at least the buttons) are good quility. It is likly to work out well for site with low traffic as well as they garente extra traffic in the first month otherwise you get 10 credits which, means 10 hits
Bottom dollar network	This is annother unique Network they say you will get extra traffic and money. But it dosn't quite work out that way, I got $0.67 for having it on my site which is not enough to qualify. Even though quite a few people used it. I would not segest putting them on a page with meny hits, maybe put it on a seperate page (I moving it to the search engines page) this way it is treated like a resource rather than an advert
	Have you seen the little clicktrade logo on any sites? If you click on it, it goves a list of the pages the link to the site you were on. It also gives you the option to link to them and tells you how much you will get per click through. This is a very good service but you have to find the right the site to get the most clickthroughs which could take some time
Registar-it / Web site garage	Yes these 2 companys share the same program which is usefull as it makes it more likly to to reach the minimum amount required. But on the other hand they work on a commision basis and though there service is very good (especially website garage) they offer free versions of there service which are almost as good. So all I have got for being a member is an E-mail each month saying "you have ernt $0.00 this mounth" But this is also a resource so if you treat it as one rather than an advert it dosn't matter as much
New exchanges	New exchanges have to be good and offer somthing new or they will not work, Both Traffic X and Multi-exchange are new and both offer unique services. I'm sure sombody will copy Traffic X's idea with a better ratio like 5:4