First off, download this map:
It's called The Gym, and it's meant to improve your accuracy and speed-shots. Just use the DMR on all of them if you wanna train for SWAT.
Now for the tips:
-JUMP. It throws your opponents off and you'll get a height and angle advantage. This does take some practice to get right, so practice with some buddies or on dummies in Custom Match. Trust me, you're gonna wanna be able to do this. It's probably the most useful skill in SWAT, or in getting headshots in general.
-Crouch when shooting long-distance. Crouching when shooting the DMR reduces the recoil.
-MOVE. Never EVER stand still. If you're searching for enemies, move in awkward patterns until you find one. Then you should stop, or you won't be able to focus on your aim very well...
-DMR > Magnum. Although the Magnum has a greater rate-of-fire, the DMR is ALWAYS more accurate, and that's what SWAT is based on.
-WHACK. Melee still does damage in SWAT. If you have shot the enemy a couple of times and he's in your face, just freakin melee him! Don't get caught trying to get some insane close-range headshot, because you're not gonna win if your opponent knows to melee you.
-Camping won't get you far in SWAT. For one thing, the DMR is not a Sniper Rifle. It's accuracy isn't that great and it takes awhile for the accuracy to return to normal from recoil. Secondly, if you're hiding behind a corner, waiting for someone to come around, you gotta freakin hit 'em in the head, or they're gonna turn around and splatter you against the wall.
-Now, just because camping won't work well doesn't mean location isn't important. First off, don't crouch in front of a rail so that just your head is sticking out. You'll think you're well covered, but what you've done is exposed the only spot they need to shoot you in. Just stand up, since you need to MOVE around anyway. Secondly, height is a big advantage. If you're above the enemy, more of their head will be exposed and it will be easier to shoot. Plus, when you're higher up, less or your head is shown and the enemy will be scrambling for a better shot.
-So let's say you run into 3 enemies. Normally, you'd be screwed, BUT THIS IS SWAT BABY. There's plenty of situations where I've wiped out all of the 4 players on opposing team, by using this one strategy: ONE AT A TIME. Just focus on the guy in front of you, because he's gonna have the best shot at you and could possibly come and melee you. Remember: MOVE, JUMP, AND WHACK. Most of the time, you'll get at least 2 of the guys, if not 3 or even all of them.