The best of traffic there is. Free! Concentrate on making Google-friendly pages. Other search engines are important also, but Google should be at the top of the list for anyone.
2. Affiliate Program
Happily pay many affiliates every month. They work hard for you, and you should have no problem sharing the profits. Building a strong buy quality traffic affiliate program is a critical element to really increase your profits.
3. Opt-in Mailing Lists
By capturing as many email addresses as possible with your newsletters, it can generate some very nice, targeted traffic.
4. Pay-per-click Search Engines
Paying for targeted traffic is worth it because you know exactly how much per click is profitable for you. Here are some great choices: Google AdWords, Overture, Kanoodle, Findwhat, and 7search.