There are some important points to reflect upon which this article addresses:
1) It is advisable to choose one or very few niches to promote initially, to enable to concentrate your efforts in few specific areas. Avoid signing up with a large number of programs which would tend to diffuse your efforts and focus, leading to mediocre results.
2) Choose your niche(s) carefully. The gravity should be high, and the sales trending upwards. The products should be of high quality, and properly priced, with commissions no less than $30. The affiliate program should provide excellent marketing tools.
3) Your website should be designed around your niche, and include quality, and original content.
4) A promotion and adwords budget is important. It is unlikely than organic SEO listings will be sufficient to quickly attract focused quality traffic, with high opt-in and conversion rates. You will have to spend money on advertisement media.
5) Submit articles and posts to blogs, and social/bookmarking sites, with your own links. You can create backlinks this way, which help search engine rankings, and "free" traffic flow to your website.
6) Submit product reviews on blogs, including your affiliate products with your affiliate and website links.
7) Building an e-mail list is important. As an incentive for prospective customers, include free downloadable items such as e-books, training videos, and software. Your list can grow rapidly this way.
8) Provide contact info on your website, and be available to answer questions from prospective customers. This builds credibility.
9) Try to recruit sub-affiliates, if your affiiate program rewards you for this effort. This will increase your commission base substantially over time.
10) Have a plan, with detailed strategies, and visible goals. Keep your efforts focused. Persistence is the key here.