Mironov’s Roadmap Theorem #1: you can’t put something new into the current development plan without taking out something of equal or larger size. When stated this plainly, it should be as obvious as the law of gravity.
It's so obvious... yet ignored every day. You can't add to the roadmap without changing the roadmap. You can't change requirements without changing the date. Read Rich's article for more.
What techniques do you use to manage change?