Most people have fiqured out that in Onslaught mode, pressing the Flak Cannon? key (the "SwitchWeapon 7" command, bound to the 7 key by default) will select the Flak Cannon? or the Grenade Launcher, depending on which weapon you have. If you have both the Grenade Launcher and the Flak Cannon?, every time you press the key it will switch between the two weapons.
However, if you don't mind manually editing your user.ini file, you can setup keys to directly select the Grenade Launcher. The following command will select the Grenade Launcher:
GetWeapon Onslaught.ONSGrenadeLauncher
You can assign these directly to keys as in:
E=GetWeapon Onslaught.ONSGrenadeLauncher
or you can assign it to an alias and assign the alias to a key, as in:
Aliases[32]=(Command="GetWeapon Onslaught.ONSGrenadeLauncher",Alias="GetGren") ..Further down... E=GetGren
What I Love to do with the Sticky Grenade Launcher is as a defense to a Goliath Tank. Hide and wait for the tank to go by, then fire as many grenades as you can onto the tank. (The best way to do it is to either stay hidden or to run right up behind the tank where it can't get you with it's guns). Once you have the grenades in place, step back and detonate! Follow up with the Flak Cannon?. If you do this while someone else is distracting the tank, you can take it out fairly quickly.
Like HoMeR says, the grenade launcher does damage pretty damn quickly. This makes it ideal for taking out nodes quickly. Note that you don't need to fire the grenades directly at the node; cover a vehicle - Scorpions work well - with grenades, then drive it in such a way that all the grenades are directly above the circle that denotes the bottom of the node. Detonating now will do as much damage to the node as if you had fired all those grenades directly at it, and you spend half as much time near the node. Have a couple of allies add their grenades to the vehicle, and you can take out a fully healthy node in one go. Covering the bottom of a manta in grenades is also a useful tactic - you can easily jump on to a Goliath Tank and detonate when the two vehicles hit to destroy it instantly.
OK, credit goes to HoMeR for this tip, as it was in one of his other posts.
Stick a bunch of grenades onto one side of a friendly Raptor (only on servers with 0% friendly fire, obviously). Then, when the Raptor is failing to dodge a missile, detonate to give the raptor a boost and hopefully push them out the way of the missile.
Another good Raptor killing trick is to place 6-8 (depending on how evil you are) mines on an empty enemy Raptor. Wait until they take off, sky high, and then detonate your mines. Also, if you die while the mines are active, the opponent in the Raptor will die too (kind of like a life-link :) ).