Work from a printout, not the computer screen. (But see below for computer functions that can help you find some kinds of mistakes.)
Read out loud. This is especially helpful for spotting run-on sentences, but you'll also hear other problems that you may not see when reading silently.
Use a blank sheet of paper to cover up the lines below the one you're reading. This technique keeps you from skipping ahead of possible mistakes.
Use the search function of the computer to find mistakes you're likely to make. Search for "it," for instance, if you confuse "its" and "it's;" for "-ing" if dangling modifiers are a problem; for opening parentheses or quote marks if you tend to leave out the closing ones.