Hence why you follow these steps
1) Find a decent vantage point where you can line up some good shots; outside of any high-traffic areas/spawn points is a good start. Improvise these places whenever possible (e.g. the tilt-a-whirl on Carnival, behind the cubicles on Highrise)
2) After getting 2-3 kills, change positions; immediately pull out your sidearm and haul ass.
3) ALWAYS be aware of your surroundings. Don't be completely focused on what's in front of you; keep your eyes and ears open for any changes in your immediate surroundings.
You're talking to someone who can sit behind the coffee counter on Highrise and get a 20+ kill streak. Real sniping is not that hard if you know what you're doing; the only maps I didn't find to be good for sniping were Vacant and Trailer Park.