I personally like to use Stylewriter pro. It is a program that finds misspelled words, improper grammar, easily confused words, overused words, weak words and passive verbs. It isn't the only tool I use, but it is a good beginning. the great thing is that after using it on four or five stories, I didn't write many passive sentences anymore. The program improved my grammar better than any teacher ever did.
I also do word searches for 'that, just, only, very, would, could, seem, had, of, and but." and I try to rephrase the sentences to do without those words whenever possible.