Stop thinking about writing things for your website. Start thinking of the people who will be interested in the overall niche that your website is a part of. Who are they? What do they want to know about? What are they talking about or asking questions about? Make sure you can actually see them in your mind's eye and the type of people they are. That will give you the initial inspiration for creating conversational pieces that they can relate to.
A great place to figure out what people are talking about or have questions about is, indeed, forums. Yahoo Answers is also a great place to find questions that people are asking. Rather than posting answers on forums, you should be creating great content that addresses those questions on your blog. Then, you can post a link that answers the question on the forum and not just the answer there. That'll drive more traffic than just an answer will.
If you sell tents, the majority of your posts should be about camping - funny stories, new camping gear, great places to camp. There may be an occasional article about tents but that shouldn't be your focus. Your goal is to gain readership and recognition as an authority in the overall niche. People who are specifically interested in your products will find them from there. Link to your main product categories from a sidebar on the blog but leave everything else (your articles) in a "general" category. As your blog grows in popularity, it will gain its own links and will pass that authority to the rest of your website.
Finally, to answer your specific question, once you know who you are writing for and what they want to read about, you'll find it is far easier to answer your own question. If you can write something every day that is compelling, go for it. If you can only do it once a week, that's fine, too. Obviously, the more you write, the more likely it is for your popularity to grow and for your website to climb because of all of the long tail keywords (unintentional as they may be). Just don't write something because you think you need to; though, only write when you have something that is definitely worth reading - not in your opinion, but in your readers' opinions.
Finally, whatever you decide, keep it consistent. Don't start writing every day, then taper off to every week and ultimately get to it only twice a month. Readers expect consistency - not only in the type of writing but in how often they can expect something new.