ProTip #1: When offered, utilize mentors
Many hackathon projects are very clever, but winners of serious hackathons are determined by how useful and relevant their projects are. Expert mentors who deal with their industries/challenges on a daily basis can give you invaluable insights. If your hackathon provides this and you're not already a seasoned expert in the subject matter, these folks are your best shot at ensuring what you’re building is the right solution for the problem you’re tackling.
ProTip #2: 99% of the time, focus on doing one thing really well
You’ve heard this before. Do one thing so well that it stands out as a shining star. Unless, of course, the main selling point of your product is a particular combination of features… in that case, ignore this ProTip.
ProTip #3: Get people to test
Beg, borrow, and steal to get user (mentor!) feedback on your interface. You work so closely that you’re bound to lose objectivity and miss obvious bugs/workflow inconsistencies that others can spot easily.
ProTip #4: Document everything
...development 101. This will save your sanity. Promise.
ProTip #5: Design matters
Demos tend to be heavily visually-oriented; the more polished your project, the more confidence judges will have in it. Technical complexity is great, but beautiful technical complexity is greater. Spend the time to make things shine.
ProTip #6: Presentation skills matter; what to ask
If you can't communicate why your brilliant product is useful or valuable to end users, you’re dead in the water. Ask a mentor or volunteer to watch you demo -- don't ask them if they think it’s good or bad, ask them to explain what your product does. If they can, score.
ProTip #7: Don’t reinvent wheels
Do existing libraries and APIs and PaaS provide functionality that can save you time? Great. Use them. Your prototype needs to be presentable, not perfect.
ProTip #8: Start at the end
Work backwards from the final product of what you'll demo -- you only have 3 minutes to show off, so make sure you build the core interface points absolutely necessary for you to demonstrate important functionality. Nobody cares about login or password recovery pages.