From Quake Wiki
The Rocket Jump
Simply have a Rocket Launcher, armor is favored, aim down, jump, then fire a rocket under to propel yourself a bit more. It is used very often, even I (CheapAlert) use it even when I have 10 health (And I die) :P
For tips on dealing with each of the monsters and bosses in Quake, see the Monsters page.
The Grenade plus Rocket Jump
Ensure that you have the grenade launcher, rocket launcher, at least 2 rocketys and adequate armour and health before starting, the more of each the better. Select the grenade launcher and fire a grenade aiming to land on a particular spot (the launch position), one way to do this is to fire vertically in to the air as soon as you have fired the greande switch to the Rocket Launcher and at the point that the grenade lands for it's second bounce move the launch position and fire a rocket directly at the ground. If you time it right, i.e. so that the rocket hits the ground at the same point that grenade explodes you should find that you are able to jump significantly higher/lomger than with just the Rocket Jump. The disadvantage is that it requires more ammo/weapons, usually takes longer to perform, which can be problematic during battles, and does more damage to your player.