In the mission Sierra 117, there is a downed Pelican at Rally Point Alpha. There is a UNSC supply box with two sniper rifles. These can be useful when you get to the dam.
After the Hunter encounter on The Storm where you must face a hammer wielding Brute Chieftain the room to the far left contains a sniper rifle. Breaking the window will allow you to snipe Covenant forces.
Because the sniper rifle is extremely powerful, but has a small combat load, it is not good practice to use this weapon against weaker enemies (i.e. Grunts, Jackals). However, the sniper rifle's penetration capabilities and superb, long range accuracy make it excellent for killing higher ranking enemies at long range like Brutes and Hunters, which, in turn, disorganizes and demoralizes the enemy troops, making combat against a large ground force easier.
Sniping fallen grenades when enemies are close by is also an effective strategy.
If an enemy will be very difficult to snipe, don't waste a round on them. Wait until they're an easier target, or just switch your attention to another target.
Have a sniper set their sights on the enemy base in a CTF match, as they can take out enemy snipers and other defenders. That way, their team mates can swoop in and grab the flag.
If you can get a couple of players to cooperate, you can get to hard to access areas on maps like Valhalla, Longshore and Last Resort and snipe without worry that close range weapons like the shotgun flush you out.
The Sniper Rifle shot can be used in close combat as a shield remover, followed by a melee which will kill your opponent.
Always take your shots carefully with the sniper rifle. If you are unable to perform headshots, always aim for the body. The key to sniping is simply getting the kill and not wasting ammunition, not getting a headshot every time. A two-shot enemy is still a dead enemy.
Always carry a secondary close ranged weapon such as an Assault Rifle or SMG. Even more preferable are a shotgun or a Battle Rifle.
When playing Team Snipers, take your shot and then move elsewhere, so your location is not given away. This is also useful in standard multiplayer.