Ducking in the shadows
If the map is right, you may choose to duck in shadows so you are not seen easily. Just don't stay in a spot too long - be sure to move around (unless, of course, the other team is clueless as to where you are).

Ducking behind objects
Hiding behind objects, other than walls, is a common way to hide-and-seek snipe. Since most objects are low, you'll need to duck behind them and peek up to get your shot. You can also crawl to a side of the object and pick off your targets.

Ducking into the open
Like strafing or jumping, ducking out into the open also makes you a harder-to-hit target. You can do this by strafing out from the wall and ducking as soon as you get into open grounds. You'll need to practice this a couple of times so you can get your shot off quickly within the sudden change of motion.

Ducking while speed sniping
You can make your speed sniping techniques more effective by ducking while in motion. Sometimes, you can get a lucky duck if your opponent is aiming high and you duck when they try to hit you. Just make an easy-to-reach key so you can duck quickly while engaged in a war.

Just ducking
There's nothing to explain here about technique, however, if you want to duck while waiting for the other team to emerge, be careful! Don't make yourself a sitting target. Hide behind objects or close to walls.