I do not agree with Dino about the run-n-gun. I can do it well, but I do not consider it actual sniping mainly because you wouldn't do that in a regular medley, but that's off-topic.
In a medley your best chance is the M82. It's more steady than the M87 and more powerful.
As for hiding, I suggest you take into account your camoflauge, since that's wht'll help you the best. SEALs usually have good camo for the map which makes sniping easier, but there are areas where a terrorist blends in quite well. Have your friend look at you and see if it's easy to notice you. Also, though you may not blend in well as a terrorist, plant PMN mines around you. You probably won't kill anyone, but you'll hear them blow up and know when someone is around you.
As stressed earlier, call someone out. If someone is on the headset at the moment, and you have a keyboard use it. Calling the person out can seriously save you.
Roofs are officially a "no-no." That's the number one expected place to have a sniper. Find a bush on the side of the road or something.
If your entire team is sniping set up a forward defensive wall and just peck each person off as they cross a line, but that rarely works since there is always at least one sniper on the other team

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I won't give out all my secrets because I prefer to keep getting kills, but I'll give you these basics.