If you're determined to do it yourself, changing the font and printing a hard copy to proofread can both be helpful. However, as some people pointed out, having a beta reader is very helpful. They aren't as familiar with the work and are less likely to subconsciously skim over mistakes; additionally, they can also spot logic flaws that the author just doesn't think of. It's also nice when they can give analysis of the characters, since readers often see characters differently than the author does.