Andrew Rollins, of TechStars alumnus Localytics wrote a great series of tips recently.
Andrew Hyde, TechStars' community director, covers many of the basics in his post. The one startup he specifically mentions, Foodzie, wrote their own post of advice, which to me really highlights how they stood out.
And Healy Jones, a TechStars adviser from this summer, has a post with application tips, but I find his post of tips for once you're in much more useful, in content and in insight it provides about the type of people TechStars wants.
The only piece of advice I can give you as a non-TechStars applicant, but Boston startupper, is make yourself known to the TechStars people. Everyone involved that I've met has been very friendly and open -- these are people who want more startups to succeed, and are just trying to find those they think are most likely to make it happen.
Best of luck!
Another tip I'd give you is to reach out to past TechStars founders. They're generally super helpful in providing ideas. They've also been known to provide a "heads up, these guys seem really smart" when they run into an interesting team. And don't over-focus on the application itself. you've got to have good stuff to go along with it, like continued progress, a great team, etc. Oh, and I run TechStars, so I'm not randomly making stuff up. ;-) Feel free to reach out to me directly at David at TechStars.org or in Boston to Shawn at TechStars.org (or Andy@ in Seattle).
Another thing to think about is showing a "path of capability". Lots of people have neat ideas, but can't get the ball rolling. Thomas Edison was dead-on when he said that genius was 1% inspiration and 99% perspiration.
Make progress happen on your idea. If you're a coder, code! If you're not a coder, sketch and design. Talk to real live prospective customers and get their feedback. Talk to real live prospective partners and get their feedback. Each day make a step forward in your idea / business. If you do that every single day, in the course of a week you've made 7 steps forward - which is tremendous. Even if you have to take a step back you're still WAY ahead of the game.
Make continuous progress. Show a path of capability so that we can say "hey, these folks have a clue and are working hard to push the boulder uphill - cool!".
[Disclosure: I run TechStars Boston, so as David suggests, reach out to past founders - as well as mentors - to get other perspectives too!]