Re-negotiate the estimates and timescales. Make sure you are the one who says how long something will take, and don't succumb to any "well, we need it done sooner" or "how about a stretch-target" suggestions.

Read Joel Spolsky's article on estimates, which basically advocates breaking the piece of work into small chunks, and estimate each one. If any of them are counted in days, break them down further until you have everything estimated in hours.