First of all, a sniper isn't going to work for every situation. For the typical, short, walk-on games or rounds most people play, fielding a sniper is impractical. Even in longer games, you may have to make some accommodations to field a sniper.
In the military, a sniper usually hikes out into the field with a squad, breaks off at some point and inserts. The sniper conceals his ghillie in a pack and suits up in the field with the squad under deep cover, or sometime after break off, again, under deep cover. If at least a couple of other players are carrying packs, the sniper's pack is less conspicuous. In scenario games, many players carry packs, so a player isn't automatically detected as a sniper just because he's the only one carrying a pack. In games where I knew in advance I would play the sniper, I carried my ghillie in an ALICE, but since almost everyone else was wearing a pack of some kind, that didn't alert anyone to my role. In games when I was asked to play unexpectedly, I have a couple of sniper veils and camo paint sticks that I make do with.
Now, if you're playing shorter games, the team(s) can still field a sniper if both sides agree to allow the sniper(s) to pre-insert a few minutes before everyone else, and then start the game. Some people have an issue with this because they think this is an unfair advantage to the sniper, but in the real world, it is common for snipers to pre-insert as much as weeks before regular ground forces, to provide recon, secure an AO, or just get into position to wait for the enemy. If people are opposed to the idea, have them at least try it once, I think they'll find it adds a lot of tension, a unique challenge, and a change of pace to the plain old capture the flag and total elimination games. Speaking of total elimination games, we usually play that if a sniper is the last man standing, it is not a win unless the last person eliminated was also a sniper, since we feel the primary objectives of a sniper are supporting the team objectives...if the team fails to meet objectives, the game is lost, and the sniper is no longer of any value. Again, the exception we play is if the last player on both teams are snipers. Time has run out in our games before the snipers found each other. If the teams in the dead box realize the sniper is the last man out, they call him in so everybody can play some more.
You can also complicate things a little further by assigning priority or designated targets, and some value to those eliminations. You can use different color tapes, tags, tag shapes, a hat/helmet, uniforms, rank insignia, etc. to distinguish higher value players. Decide ahead of time what effect or value elimination of particular players has to the outcome of the game. For example, you could field two 5 player teams and a 6th on each as a sniper. Elimination of a player designated as the commander/or priority package could equal 5 points, and all other players including the opposing sniper 1 point. Set up the field of play so that each team must pass each other somewhere on the way to their objective, and also pass the sniper. The first team to their objective scores 5 points. The first team to 5 points in any situation wins. There's a few ways to get to 5 points and win, so the strategy changes from game to game. You can see the potential tension being a little flexible and creative can add to woodsball play. Have fun...we do!