Time-driven development
Young hacker looks at the figures: “2 hours for the feature Foo, 4 hours for the feature Bar”. He feels that kind of pressure: “I have to make it! I have to type faster, think faster, test faster.”
This is an awful feeling. So here’s the (possible) solution to this situation: try to think of time as of money you are investing. Tell yourself how much time of your life would you invest into this piece of #$@^ (of course, take into account your rate/salary). Now it looks like you score the feature Foo for just 2 hours: it doesn’t worth 4 hours or more. Spend 10-15 minutes for planning the way to spend that much time and do your best. If some trouble strikes and you’re out of time, just give up. Go to another feature and let this to be discussed on a weekly meeting when there’s time to schedule next iteration.
If the client wants a fixed price for software, you will not have any additional time. In such case - either do a dirty job, or work all the night. You to decide.