In QL you will always have a weapon out, so the first trick is to know the right weapon to have out for a given situation:
If you know your opponent will be at long range have out your rail.
If he is at mid range, and you don't mind being noisy, have out your lg.
If he is going to be close, use RL.
If he is low on health and you want to get a quick shot without aiming, use sg.
If you want to be a jerk, have out your gaunt. :P
If you are walking up to a doorway, and you might be jumped, take out your RL.
If you are approaching a door way and you have LG, make sure to look through the doorway from some distance before you go through.
OK, switching weapons. This is mostly done when you are actually in the middle of the fight. The trick here isn't really reflexes so much as quick decision making, with a couple of exceptions:
If you are LGing someone and they use the push back to dash away from you out of LG range, you can switch to rail instantly and hit them with a shot.
If you want to rocket jump and then switch to a different weapon to shoot someone while you are in midair, you should do it immediately after you do the rj.
Most of the time, however, you'll find your self in a situation where you are fighting someone and you don't like the effectiveness of your weapon. The trick here is to quickly form a simple plan and execute.
If you are fighting him with rocket and he's lging you for more effective damage, you can quickly escape and shoot him with a rail as you back up.
If you are in a rail fight with someone at close to LG range and you've both just shot off a rail, you can break out your LG and advance towards them.
The key here is to make a decision faster than the other guy can make the decision (http://en.wikipedia.org/wiki/OODA_loop), and then have the reflexes to execute smoothly.
Finally, some situations where you should NOT switch weapons.
You're fighting with rail, you've just fired a shot, and and someone with a good deal of health closes on you with LG. In this case, you can try to switch weapons, but the problem is that weapon switch can only happen after the weapon finishes its reload cycle. With rail this takes a long time. Meanwhile, the other guy is basically shooting you while you are defenseless. This is why you need to run away or concentrate on dodging. If you can't, it's better to just wait for the rail to finish reloading and shoot him again. At least you will get in 70 damage before you die.