like jackie said you need to keep your rocket trajectory low, if you come in from the front and high i can pistol you before you even get close to landing. if you can curve it around a building you will come in fast, low and be extremely hard to hit.
another boom that i have good success with is a slow rocket over a wall, you essentially move forward just enough to come over a wall and can drop in on people quick so they have less reaction time to get you. you will probably not vomit in time but you will get shot directly next to people.
don't forget to air strafe with A & D, sometimes that can save you if you have no choice but you boom from straight on