Need help increasing my positioning in keywords

I need some advice. I ranked for about 800 keywords but I only rank for about 100 keywords in the top 50. Can anyone provide me details on how I can increase my ranking on the other keywords? I have a pretty good page rank of 4 and PA 51 and DA 43. I just need some advice on what I can do to increase the rank of these keywords. Any advice would be appreciated! Thanks,