I think, as other posts have pointed out, the best thing is to use your experience to really study/analyse the requirements, and possibly even to do a good design. Sorry, the _best_ thing to do is actually get good requirements. I once waited a couple of months after a verbal spec for a short project and got a one-page hand-written flowchart. Although that's partly my own fault because I said it was the minimum I would accept before I started work on it.

My other peeve is that I often get caught at 2pm on a Monday when I've just come in from an hour's 5-a-side-football to give estimates on fixing some change requests. One day I'll get on that assertiveness course. Until then I generally play it safe and only say I'll do a couple of small things for a given week. Which often works out to be quite accurate.

If your projects do slip, make sure your boss understands exactly why, especially if you have done a detailed plan. As I work supporting software for manufacturing tests (much of which was written by people who left before I joined the company), project timescales for new things will have to include some time out to fix unforseen urgent change requests for old code, or even for the slippage of lower priority change requests. That is never easy to account for. My boss isn't a software guy, but he does understand when projects slip for good reasons.