Top affiliates take the time to learn. It always amazes me when I receive email from someone who joins an affiliate program, but then asks me exactly what it is they're selling and what they're supposed to do. Know your product. Know the website. Spend some time surfing through the site to familiarize yourself with what you're getting into.
Top affiliates are fresh and original.
Many affiliate programs provide their affiliates with pre-written emails, advertising copy, or articles. They also usually provide a number of banners that you can choose from. After you've seen the same tired old ad time and time again, it simply becomes ineffective. People will ignore it since it's been used too many times. Top affiliates provide content.