Deconstruct. Break whatever you're building into smaller features that you can implement in stages. Then, whenever you have any of those smaller pieces done and you've tested to confirm it doesn't break anything, deploy it and show it to the Powers That Be.

Using small iterations will often help you finish the larger project faster and better, because you're getting feedback as you go and you won't need to backtrack and redo as much. But even if it doesn't, you're showing constant progress, which has a solid psychological benefit and restores your manager or client's confidence.

Test-driven development also helps a lot with this approach. At first it may seem like writing tests first slows things down -- but it gains that time back in bugs you'll avoid, and depending on how you write them, the tests themselves could be a deliverable you can show to the Powers That Be and confirm the app's behavior even before you write it all.