Some quick ones:
- Make value proposition clear. Just because they reached the signup screen doesn't mean they're already convinced to sign up.
- Show social proof on signup screen, such as customer logos or approximate number of users. Same reason as above.
- Offer SSO options. Google is especially popular
- Have an Obvious Next Step(tm)(R)(patent pending). Make it clear what the visitor is expected to do next by making that thing visually stand out, and muting (or removing) everything else. Notice in OP's signup screen that the background app preview is tinted, and the Twitter login button is the brightest thing, dead-center of the page.
- Have a product people want. This is a hard one. :)