For a start, banner advertising is not very effective any more.
You might be better off having text links with decent ancor text linking to your website.
Are you paying them per lead or per sale?
I wrote a very short article on how to super charge your affiliates
http://www.marketershandbook.com/art...affiliates.htm
You might want to take a look at that, might give you a few ideas.
How about instead of just getting people to send people to your website, how about getting your affiliate to send people toyour newsletter or an auto responder telling them about your apartments and why they are so great.
Basically have a programmer create somthing that allows you to give your affiliate a subscriber sign up box and pace it on their website, then anyone from that affiliates web site is tagged with thir affiliate ID, then if that eprson makes a sale, the affiliate is credited.
This could give you a new slant on your affiliate program and hopefully get you an increase in sales.