This is one of those "how long IS that piece of string" questions, it's almost mindless in its intent. The purpose behind every-single-website on the planet is how to increase website traffic, to further extend either your product offering's or the services that you provide. Increasing traffic to a specific site is not all that difficult, as I have found out over the years as an SEO consultant for Azzuro Blu. Frankly, I know of several ways that you can artificially increase traffic (or should I say, "alleged" traffic) to a site for less than $20 per month, and I'll guarantee at least 10,000 hits per month, probably more for that price. This method (no, I'm not going to tell) is great if all you want is to show that your site can get hits, even if the hits are bought, but what if you want to attract buying, targeted traffic?
This is where it gets a little bit sticky, and a lot more costly when done wrong. If your asking how to increase website traffic that matters, the only answer that honest is that you go out and look for people that want your product. Sure, that's not as cut and dried as it may sound, but it is still true. For this service, expect to pay. Especially if you use a listing service, that guarantee's a top listing for you. What they don't usually tell you is that those "guaranteed" listings are what amounts to ad's, not listings for your site. Sure, you'll get the traffic, for a second or two, but does that justify the costs involved??
Your call on that, but there's only one way that will always return customers to you at negligible cost – content. What's on those pages you put sweat and blood into. If your site is 3 years old, frak, if only 3 months old, and still with the same intro page text, your losing out – massively. With fresh content, your pages will continually come up at the head of the search engine results, once you have the correct tags in place to denote the content ('tags' are those single words that are in the body of the posting that will bring the content up when searched for) and if the content is relevant.
Bottom line is that it doesn’t need to cost Earth when it’s down to how to increase website traffic, it just needs good stuff that people want to know about on your pages