What do you think is the most important source of traffic, and which should you give more effort in improving? I'm looking at my traffic stats, and the breakdown is as follows:

Direct Address/Bookmarks 53.7 %
Links from Search Engine 12.9 %
Links from External Sites 33.2 %

Which is the most reliable source of traffic out of these three? Should I concentrate more on search engine optimization, since this is more important and I've got a low percentage of it?

Personally I feel that links from Search Engines are the most important. You should deffinitley focus the main majority of your efforts on perfecting that. I've learned that at least 80% of all web users will stumble apon sites through search engines, rather than the other numorous ways that there are. Then again, that is my oppinion. If you need some marketting ideas, I'm always around to help out with that.