[annnnd]

I find it much easier to estimate the time it will take if I take myself out of the picture. When estimating, I always try to think "how much time would it take for my colleagues to plan, execute, test, etc... something"? It is almost impossible to judge your own speed, but with experience you learn how to estimate speed of others quite accurately.

Of course, I also let product managers know that any change will affect the deadline, and almost never in a good way. I do this every time they have another "really simple idea".