First of all, you obivously want to be relaxed in person. Playing the game right before an exam is not the greatest idea for accurate shooting - which is the most important when it comes to sniping. I actually watched Shooter yesterday, which gave me the idea of this entry.
Use the advantage of a sniper! You can aim much better than colt's, deagles and AK-47's from long distance. Take advantage of this, find a spot there is far away from everything, but where you still can see the enemy coming. In de_dust2 a sniper is essential for holding A-long or even middle at the first 10 seconds of the game, praying for a spawn kill of a slacking enemy.
Full zoom is the preferred, but you can also use semi-zoom. Works too. ALWAYS have a nice offhand, such as deagle or USP while holding a sniper. Remember, while being zoomed you can see all the stuff around you, and the enemy might sneak up upon you.
Here are some good sniping spots I've drawed up for you. The purple star is you, and the red line is where you wanna be aiming. This is both for playing counter-terorrists and terrorists.
And to finish this entry off, after taking your shot, quickly press Q twice, so you go to your gun and then sniper swiftly. This is faster than reloading.