Pick an area that will always be popular, or something futuristic.
If you pick something that will be popular all the time, then chances are there are sites that are already up that you have to compete against. Not saying it is impossible but a bit harder. The real benefit is when you start to out perform others in your niche. Without giving to much away Entertainment & Sports works well for me.
Now this is something new I am going to attempt. I am targeting something in the really far future. Although it might not happen I am hoping it will in which case I could build enough backlinks, articles and discussion to totally own the niche. Again I don't want to do all the thinking for you however I will give you an example. Lets assume (eljs favorite :P ) that no site existed (although it does) and they find aliens on Mars in a year from now. In the whole year you could build up all kinds of stuff on your site. By the time the event happened you will get tons of traffic.
Hope that gives you some ideas