Scoping
First off don't stand in the same place to long. If you do kill someone the chances are they will want to get you back and if your staying in the same place then your going to be an easy target. So basically move around alot.
Secondly, don't stay scoped for too long if you can't see someone go to another position and always watch your back and never go scoped in the open unless your confident you can get down scope and kill your target in one bullet(i.e. get a headshot) within 2 seconds, any longer your a sitting duck.
Lag shooting, you may have heard about this and may know what it is but if not heres an explanation [This is a really handy technique to have btw] When your firing at a moving target rather than shooting where they are at that moment aim just in front of them and fire. Aim for the shoulders/neck level and most times you'll get a headshot. And if you don't kill the moving target straight away keep trying, remember it only takes two body shots to kill someone with a sniper so you only have to hit him twice and they die.
Another technique which comes in handy, particularly when your fighting an able sniper is the ability to kill them really quickly before they can kill you. I'll use an in game example here, when in the Port and your on top of the 3 story building and your sniping at a chappy in the small room, stand in the position so that when you tilt your nunchuk you can just see into the room as you do this get your reticule lined up on or near their head. Do not move your pointer at all[press 'c' to lock the zoom], lean your nunchuk back in (so the other sniper can't see you) then really quickly lean it out and fire, it's a really fast kill, your opponent can't react and you have a nice little headshot to add to your collection.
Furthermore, when you go scoped analyze all the other positions you can think of where snipers could snipe you from. Quickly check them out to see if anyone is there, if not you can afford to look out into the open where you may be able to snipe one of the little SMG noobs, but remember to lag shoot. If you do get shot at and your screen gos red the retreat instantly duck down or run away from where you were standing into a position where it is impossible for that sniper to snipe you again.
If you can see someone standing still, the best way to kill them is to scope there head, rather than gently twisting the remote to zoom in quickly rotate it 90 *degrees* clockwise so that your instantly zoomed in as far as you can go, as fast as possible then you just aimn for the head. Al;ternativly you could semi-scope them by just aiming half the way down, but it's good practice to try and get all the way down accuratly and quickly. (B3nlamd is awesome at doing this)
Also try if when your aiming at a sniper who is firing at you as you walk stay scoped (only do this is your playing a noob sniper, or you get really good at the technique) it will make it alot less accurate because your scope will constantly be moving but if you time it right you should be able to kill the sniper and he should have more difficulty killing you because your moving. Try going side to side when doing this.
No Scoping
Greatest technique ever! To start of with shoot at a wall (in a private locked server perhaps) aiming for a specific target like a light. Soon enough you will get the jist of where the bullets fly.
Then if you don't want your score to suffer, go into an unranked game on a small map (not sewers ... grenade fest ) and only try your no scoping out against other players.
Your gun should look like it is to the right () of the target when your fighting them and aim slightly to the right of your targets chest (usually results in a head shot). Don't go to close to the target when your fighting them but no to far away, eventually you will find a happy medium.
The best way to practice is to build up. Go into unranked games and then when your confident go into ranked Sniper only games and only when you think your no scoping is amazing (2-3 bullets a kill - thats for DM's anyways ... I'd say you could probably afford about 5 bullets a kill in the other modes because you have team mates to help you out) go for the normal [all guns allowed] servers.
Before long you will be racking up the no scope headshots
Finally a top piece of advice, use the deathcams! When you die quickly press along on your mote so that you see where each of the players is, then try and remember where they are and when you spawn you can go and kill the people you remember where near you from the deathcams.
Good gewehr at start of vid:
YouTube - JonyPapi´s montage MoHH2
wb, SC0TTYTEC