The Internet is chock-a-block with tips on how to make sure that your vote is counted, whether you vote early or on November 4. (Sorry, no early voting for Marylanders, but the issue of whether to allow early voting for future elections will be on the ballot.) Some solid advice comes via Myra Armstrong's Huffington Post article, "Easy Steps to Ensure Your Vote is Counted."
One thing this and others have noted is that you should take a camera with you, if possible, to record possible problems; even Oprah thought she had a problem having her vote recorded, and her bff Gayle intends to take a snapshot of her vote just in case. Because many camera phones have video function, this tip could really come in handy; if you notice any problems, you can then upload the video to videothevote.org or youtube.
An extra piece of security to consider is to get a sample ballot (I'm getting an extra from the library), mark in ink how you intend to vote, and date and sign it--preferably in the presence of a witness. But I'm just paranoid that way...
Wear comfortable shoes, take a good book and music with you to the polls (if you haven't voted already), and have a great 4th no matter what!