On one hand, long and ugly affiliate tracking links are a straight giveaway to people that you want to sell them a product (and people may be uncomfortable clicking on them). Therefore, making your link shorter and "nicer" looking will it give a more professional appearance. One of my favorite plugins for making links look "pretty" is the Pretty Links WordPress plugin.
Create a website about the topic that you have chosen. The key to succeeding with affiliate programs is to build a content-rich website with useful information built around a specific theme. Don't just lazily throw up a quick site or blog and string a bunch of affiliate links all over your pages. You have to start by building credibility, and the best way to do that is to write about something you know a lot about.
Always disclose your affiliate relationship. Most visitors will probably understand that graphic ads will lead to your getting paid, but if you write a review or use an in-text link as a recommendation, you want your readers to know that may lead to compensation as well. This ensures you retain transparency and trust with your readers, but also, it's required by the FTC's endorsement rules.