The rule listing thing i made with tanks worked well, so i'll use it here aswell.
Rule #1: Scout first, Sniper second
What I mean is that your primary role is reconnaissance, and then owning people from long ranges. Use those damn binoculars to spot enemies and give targets to artillery. That way you get one helluva score, if the arty guy knows what he's doing however.
Rule #2: Movement kills.
You probably have killed someone that you haven't noticed until he moved an inch. That is becouse human eye is attracted to movement. Now that you noticed how easy it is for enemy to spot you, then don't move. Stay in cover, and if you have to move, cover to cover is your best friend. If you can, run in a ditch, so that you can't be seen until the enemy is above you.
Rule#3: Sniper roles.
Sniper roles can be divided to two categories: offensive and defensive.
Offensive sniping involves providing support for your teammates, including tanks. See an enemy AT gun that is raping you tanks? Snipe the gunner. Oh you can't hit him from this angle? Then spot him for arty. Is vonMudra raping your team again with the lafette? Snipe him then. Note that even though you can snipe people from artillery guns, at certain maps and servers it is not appreciated if the guns in question are in uncap bases.
Defensive sniping is a bit more challenging. If you notice a flag is constantly being attacked from an unexcpected direction, there's most likely a squad leader, which will be referred as SL from now on, hiding somewhere nearby. See where the enemy is coming from, check your map for the likely hiding position of the SL, then flank it and observe. If you see enemies popping there from thin air, you know you're in the sweet spot. Now figure out his precise location and kill him, then you can make merry with his squad members. Other thing about defensive sniping: enemy machine gunners and SLs are primary targets. Prevent the enemy machine gunner from suppressing your teammates by killing him.
Rule #4: Use your equipment.
Mine the most likely route the enemy will take, but most unlikely route your teammates take. That way you won't get the tamkills so often (hopefully). Use smoke grenades to cover your movements and prevent the enemy from seeing your teammates defensive positions. They'll have to go Leeroy Jenkins through the smoke and get raped by a defending lafette vonMudra. Binoculars, see rule #1. Knife if you think you're ninja enough. If not, then use the pistols for headshots. Suggestion to devs: Hi-Power for Canadian sniperkits. Please?
Rule #5: Know when NOT to shoot.
If you are heavily outnumbered, don't shoot, you'll just get their attention. You don't have to kill every single possible enemy, only the biggest threats to your teammates, IE gunners on MGs, AT-guns and mortars, SLs and anti-tank troops. If you think you've been spotted by an enemy, take him out, s-mine and relocate.
That's all for now, don't bother asking for bullet trajectories or other stuff like that, practice makes perfect.