Hmmm...well, I'm not exactly sure what I do when I snipe....I've always been more comfortable with sniping than any other type of combat, not only in Mechwarrior, but other games too....I'll have to think about this, so don't be surprised if I miss something....
1) Know your weapon ranges. Nothing worse than launching a gauss at an unsuspecting enemy mech, while they're out at 850. Not only did you just waste ammo, but you probably just put out a neon sign that says "Hey! Look! There's a Sniper over here, come kill me".
2) Know your mech. Know where your weapons are located. For example, in a Clan Gauss, ERPPC Chimera, both your guns are in your arms. This'll help when you're jumpsniping (or poptarting for that matter). When you jump, you don't want to shoot untill your arms get above the ridge, or obstacle you're jumping over/above
3) Know your enemy's mech. For example, you'd want to hit the Nova Cat in the arms, and strip off their weapons. This is because the majority of the weapons on a Nova Cat go in its arms. And it's fun

Nothing more satisfying than seeing a Nova Cat running around screaming, "Kill me! I have no weapons". And if you're feeling particularly mean, leg him, then tell your teammates to leave him alone .
4) Know your role. If you're sniping, you don't want to rush in gungho. You're gonna want to stick back, and well....snipe. Rushing in will just get you dead, and that's something you want to avoid (unless you're me, then it just happens no matter what anyway

No offense to anyone if they are, I'll use it occasionally too).
7) Always use ECM/BAP. If it's available take it. These two electronics are invaluable. The ECM will keep you hidden, while the BAP will pick up any mech within 1200 meters (ECMed mech at 750). If you have Light gauss, you can make great use of the BAP. The Light Gauss can shoot out to 1200 (you may not be able to target them, but you can still hit them, just aim far enough ahead of a mobile mech to lead them. And you can tell ig you hit them, by going to the Damage overlay, and watching the picture. If the mech rocks back and forth, you hit him. If it didn't, then well....you missed, or the slugs hit an object before the enemy).
8) Passive radar. If you haven't got ECM, and you're out on your own, you're gonna want to be passive. It'll keep you protected just the same as ECM, except you wont be able to see anything that's not within 250 meters. If you're passive, you want to keep a good eye on your surroundings, and Nav points. You don't want to get ambushed by a mech coming in, without being able to see it. Passive radar is also good when you want to stay hidden, while you're moving to set up a surprise ambush of your own. It'll keep you hidden from a BAP equipped mech up to 450, rather than the 750 they would normally see you at.
9) Pay attention to your enemys weapons. If you see a mech tht has only short range weapons, make it a priority target (assuming you're not dealing with a mech that's closing in on you). You'll be able to shoot them, without being shot back. It's a "Safe Target" in this case. Though, that's not the only reason you would want to see what the weapons they;re carrying are. You'd also want to stay away from a mech with heavy hitting short range weapons.
10) Only fire when you're comfortable with firing. If you think the shot will expose your position, don't take the shot. Your job isn't to kill, but rather to wound, and leave for your teammates to take care of.
There is a bunch more things I'm sure, but most of it is just learning. Take your time, use cover, fire when it's safe, and I'm sure you'll do fine....seems to work for everyone but me....I'm just a natural target....maybe it has something to do with talking so much