The main trick I use, and try to teach to everyone I play with (in person, anyway), is that you can switch weapons as soon as you put a new magazine in when reloading, and it still counts as a full reload. You can reload two different weapons pretty quickly this way.
For example, with the rocket launcher, as soon as you see the Chief slam the new rocket pack into the launcher, switch to another weapon, and you save yourself a few seconds. :3