While I haven't had much luck winning lately, I have a little advice. I'm not sure who I got this from. My two main sources when it comes to entering contests, are Sweetie's Sweeps and About.com contest columnist Sandra Grauschopf.
One of them, I'm leaning towards Sweetie, said to space out you entries on blog contests. So even if you do everything at once try to wait a while before you put that down. Unless the rules don't permit that. She also said to wait until after fifty people have entered, because if they are using random.org it usually chooses numbers higher up.