Prepare beyond optimism.

Optimism is a decent attitude and a poor plan. If your arrangements only work when things break your way, you have not built anything; you have made a bet. Preparing past what you expect is how you keep your hopefulness from being load-bearing.

Take something you are counting on and ask what happens if it comes in twenty percent worse: less money, more time, fewer people helping. If the answer is that everything falls apart, add margin now. You can stay hopeful about the outcome while making sure your position does not require it.