Hi there
It depends on the niche the site is in really.
If it's in the MMO ( Internet Marketing ) niche then Jvzoo isn't a bad choice and free for first product.
and there is also Clickbank of course.
If the site is selling non digital items then CJ ( Commission Junction ) might be worth a look.
As for tips:
Offer a decent % and earning potential to your affiliates and provide them with plenty of material( swipe files, banners, reports, videos, articles )to use in there marketing. Maybe provide ongoing free material to send out to there lists.
If you look after your affiliates, then they look after you.
Good luck!