After your first draft is written, print it out and highlight every fact that you listed. Go back to your original sources and verify those facts again. This is a step that is often skipped, and will often cause the most trouble. While these programs are hardly foolproof, they will catch the most egregious errors. Now it's time to read your copy aloud.
Some people swear by highlighting text sentence by sentence or line by line and reading it that way.
Start at the bottom and read the words individually, and read phrases to make sure you are using commas or other punctuation correctly. You can also check for article agreement and verb agreement. Reading the text backwards takes the words out of context, so you're more likely to pick up any misspellings you may have missed or any punctuation that doesn't make sense. The absolute last line of defense is to have someone you trust read your copy. Pick someone who is a stickler - I've found the most annoying copyeditors are often the best because they point out every little thing that is questionable.