A large part of being successful in the search engines is keyword competition and popularity. If you choose keywords that have a too high a rate of competition it will be nearly impossible to get ranked high. Choose keywords that are not popular enough and you'll get high rankings, but little traffic from them. You need to find keywords with low competition and high popularity rates.

I personally use a very dynamic tool call WordTracker. You enter a keyword that you're looking to get traffic from and WordTracker does the following. First it gives you other keywords related to your main keyword. It also gives you an average of how often these keywords are searched for. Then it shows you how many websites are competing for this keyword. It allows you to find those keywords that are low in competition and high in popularity so you can attack them when doing your search engine promotion. WordTracker will give you new ideas if you're frustrated with search engine marketing.