Hide where nobody expects to find you. For example, ever see a sniper hiding in the water in 2fort? Most people dont until it's in their kill cam. I like to constantly change positions, once you kill somebody a couple times from the same spot they're going to get angry and just go scorch shot pyro and ruin your day.
i tried maining sniper once, i usually aimed at the wall and waited for the enemy to walk accross my crosshair and flick to the head, or let my crosshair be in the head region and wait for the head to come.