A Good Sniper needs to... 
be aware of his terrain 
be able to judge distances 
be able to connect to the target (wind velocity, distance, and movement of target) 
be patient and calm in high stress situations. 
be able to move across terrain without being seen. 

The truth about Snipers 
    Hollywood has painted a pretty noble picture for the sniper. In the movies a sniper will simply track down his target and "single handedly" eliminate it. The truth about snipers is that they actually consist of a two man team (the shooter, and spotter). The team goes through a very thorough plan that is formed from all the intelligence info that has been gathered. Reaching the target is not always that simple. Usually a sniper will be air dropped (via boat, submarine, or parachute) several miles from the target and actually crawl the last kilometer to their nesting position (it can take a day or more). Their weapons and equipment are contained in a "drag bag", pulled along behind each. The spotter will cover their tracks and make sure that they are camouflaged at all times. Under the cover of darkness, they carefully, methodically carve a hole in the ground, then re-cover it so artfully that it seems perfectly undisturbed. In this hole the pair will live for a day or two, observing, recording, and possibly reporting by radio.  When the team arrives at their strategic nesting area, The spotter uses a compact telescope to look downrange. Instead of watching the target, the spotter watches for the bullet as it streaks toward the victim. With proper training and the right weather conditions, you can actually see a bullet and the vapor trail produced as it flies through the atmosphere. The spotter reports the point of impact to the shooter for any corrections.  

Simple Sniper strategies for Delta Force series 
1) They most important thing to remember when sniping is to move behind cover after shooting. Too many inexperienced snipers find a position they really like, and stay there. This will give them one maybe even two easy kills, but after that the sniper becomes LAW bait. 

2) Remember! it's hard to hit a target that runs from left to right. Get behind the enemies traffic zone so that he's running "to and fro." 

3) Be aware that your weapon does make noise, If an enemy straggler hears you just over the hill guess what. He's coming over the hill. 

Other points- Some games are more conducive to snipers than others. For example, snipers on the defensive team in A&D games aren't as useful as snipers on the attacking team. Snipers in S&D games are utterly useless. They neither attack the enemies base, nor defend their own since they are never there. Otherwise, snipers are a good commodity if played correctly, which includes communicating with their team since they usually have the advantage of high positions and can see more. 

Advanced Sniper skills 
Posting a couple of claymores behind your sniping position (pointing away obviously) will help keep others from sneaking up from behind since claymores are difficult to spot in tall grass. 
 

A real sniper team consists of a Sniper and a Spotter. The Spotter is usually armed with an assault weapon such as the M4. In the game of Delta Force 2, a spotter can only act as rear and flank defense. Any forward fire from the spotter will reveal the sniper's position. The Spotter must keep his fire limited, as it is usually revealed by tracer and flash. If the team is spotted then an evasive move should take place. The Spotter should follow and defend the Sniper. When a new nest has been settled, the spotter should place claymore mines  around the backside of the nest. 
 
 

A Sniper must cover himself well. His rifle does not make a good close combat weapon, so it is vital to stay away from any close action. At times the sniper may be found by straggling footmen. There are several things you can do... first step is to prevent an encounter. Find a spot that has very few access routes (usually to the rear of your nest). Place Claymore mines to keep footmen off your rear. I 

If you have spotted a straggler in your nest area, stay still, and arm yourself with your side arm or secondary weapon. Be prepared but don't reveal your position if at all possible! You must be prepared to attack or defend. If the straggler is just passing by, then you may choose to pursue him with your knife. The problem with this is that if you kill him he will now know the area of your nesting. You must then choose a different nesting area (far away). This will cost you time and limit you to a small amount of kills. More foot time equals equals less killls. 

A sniper should always be where he is not expected. Ask any foot soldier where he first looks and he will tell you "the high ground!". That doesn't mean don't use the high ground, just don't be obvious about it! I cant count how many times I have looked towards a hill peak only to find a sniper with no clue. One time a sniper put his nest where it could be seen from the respawn area. He killed me, but I saw him. When I respawned I just crawled through the grass till he was in my sights, and I shot him at 450 meters. He thought he was pretty crafty till I put a round through his scope.