The classic answer will be to slow down and look ahead. I myself had the same problem, and was skeptical to the answer above, but I tried it out.. now I've gotten some sub 30 averages (still working on consistency). Whenever you feel the need to speedcube, don't bust out your timer and do an average or whatever. instead scramble it and work on your f2l. Move slowly and look ahead, track pieces, try to move at a constant rate without stopping. Eventually you will get faster.
There are many, many, many threads on this subject, all with the same answers you're going to get here. Please spend some time looking through the older threads and reading through other people's postings.