If you try to sell a product that is in low demand then chances are that you are not going to get many sales no matter how hard you try. So it is a good idea to spend a bit of time researching and finding out if a product that you are thinking of promoting is a product that your audience needs. If your site gets decent traffic then you can conduct an online survey and easily get input from your visitors.
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.