If you're sticking to a single category, branch out and try some of the others. Instants are popular because you can find out if you're a winner right away.
Cast your net wide, but be selective (I know, that sounds contradictory). You may be focussing on the big stuff, but don't overlook the small stuff. A book or a DVD isn't going to pay your bills, but its still something that may show up in the mail instead of one. The little stuff adds up.
Make sure you're not just checking your inbox, but also checking your spam folder for winning notifications (some of the same things that flag an email as spam are frequent traits of winners... comes from an individual you don't know, has an attachment, etc.)
Unless you're only entering the stuff that's about to expire, a lot of the sweeps you've already entered won't be done for weeks or months, and even once they end, sponsors like to take their time notifying winners, drawing second chance winners if the first round of notifications flopped, "handling" the prizes, and shipping the prizes. It could be anywhere from 6 weeks to 6 months after a sweep ends before the actual prize shows up. This is a patience game.
Some sponsors don't notify, you may have already won and won't know it til something shows up in the mail.