if you know theres another sniper jump out and keep straifing back and forth tell they shoot gives you about 3 seconds to scope in and hit them in the head really quick works most of the time if you straif fast enough they wont hit you
I agree, your first step is to make your opponent miss, unless you are already out in position, then snap a shot first and start dancing till they miss.
If you get dropped, charge up before you come back out, strafe out and aim head high so you can take your shot ASAP. If you're up against multiple snipers....you have to dance like a mad man after your first shot. Don't forget to duck/jump now and then too.
I, myself, only charge up when there is a lack of targets. Otherwise I am constantly going for head shots.