6. Be independent in your topic selection. Reading through lots of Top 10 lists (and I've read them all, for my Top 10 GameFAQS Top Ten Lists list), you'll quickly notice that many of them (especially the ones from 2007 to 2009) tend to follow a specific formula. There's a vaguely interesting and ill-defined idea. There's 4 or 5 popular games that the writer manages to squeeze into the list topic. There's 2 or 3 games that legitimately fit, and then there's 2 or 3 games that the author gets really excited about.
The reason for this is that oftentimes, someone will write a list solely to praise one or two games. Sometimes authors will even come up with a topic solely so that it'll put their favorite game at #1. That's very bad. In writing a list, you're not writing about individual games; you're writing about a topic as a whole, using games to explore it. If you're more interested in the game than the list topic, you probably shouldn't be writing that list.
Now, that's not to say lists can't be inspired by individual games. My Literature-Based Games list was entirely inspired by Dante's Inferno. But the key is that the list is not solely motivated by that game; that game introduces the topic, but the list doesn't exist solely toward talking about that game. It just introduces an interesting question. It's a question only you can answer: are you writing the list for the topic, or just for a particular game or two?
7. Go Outside the Box. Most of the advice I've given thusfar has assumed your list is based on a game: but the great thing is that nowadays, SBAllen accepts lists that aren't necessarily rooted in a single game. Some of my lists include Events Where Gaming Impacted The Real World, Game Companies/Franchises/Designers, and Voiceover Artists. Other recent lists include Console Features, Crimes Committed By Mario, and Best Things Bungie Did To Halo.
So if you have an idea that isn't necessarily rooted in one-game-per-item, go for it! Those are oftentimes the best lists of all. There are other things you can do to break the normal list framework as well. You can have your list motivated by actual numbers, like polls, mentions, or sales figures. Instead of ranking games ten to one, you can use those ten items to represent years or genres. You've got a lot of leeway to choose your topic nowadays, so take advantage of it.
8. Get Feedback. We're here to help, and the board sticky clearly prohibits stealing ideas. So if you think you've got an idea, post it here. We'd be glad to help point out its flaws, its strengths, and potential traps you might fall into in writing. We'll help make sure you've got some formal criteria, and that you know what you'll need to explain. So by all means, no matter how preliminary your idea is, post it here.