Don't overload your site with advertising.
Look for programs with good, frequent reporting. Similarly, investigate how referrals are tracked and if the technique is reliable.
What is the minimum payment? Too low and bank fees will destroy earnings, too high and you may wait years for your first payment.
Maximise your income by diversifying the types of programs on your site.
Use "exit point strategies". Find out which pages people use to exit your site. They are looking for somewhere to go, so place relevant affiliate links on these pages.
Some affiliate programs allow you to generate additional income by signing up other Web sites - this can be a percentage of the new site's income or a finder's fee. Beware of pyramid selling schemes, and check with the Australian Competition and Consumer Commission (www.accc.gov.au) if you need more help.
Consider the ethics of mixing advertising links with editorial.
Frequently check your links - if your link is incorrect or broken, you may lose sales.
Make your own banners for your site based on the affiliate program or product - but watch the area of copyright, particularly if you are going to use artwork from a book or CD.
In addition to targeting products, another important consideration is increasing visitor traffic.