Sam JTB, thank you for joining in with us and contributing.
I think the fastest way is ppc/adwords. If I have a budget to spend, I can't get any faster than that. SEO takes time, but may be the most effective. You won't get recurring traffic from posting in forums and blog commenting. With ppc and seo it can mean a stream of visitors every day. Depending on the popularity of the keyword that could be 20 visitors or 2000. SEO does take time, but worth the effort if done right because it is a fixed cost vs a variable cost associated with ppc over time. If looking for quick traffic, do your keyword research. Setup a ppc campaign with a budget, then get to work on SEO. Posting in blogs and forums can facilitate this - and the more popular the blog/forum then the better/quicker the results you will get. hth