When snow conceals the trail, scout for other signs. Blazes cut into or tacked on trees, an open corridor in the forest, and a path-shaped depression in the snow are all likely indications that you're on the right track.
Novices: Stick to well-marked trails in winter.
Everyone: Pack a map and compass/GPS in case snowdrifts force you off the route. In a whiteout, use your watch's altimeter to navigate by your topo's contour lines.
Check with rangers or a local gear shop for the location of winter-only snowshoe trails. Many don't appear on maps, but are easy to follow when packed down.
Stay in the trees when the forecast calls for bad weather; snow makes it easier to get disoriented in open terrain. If thick flakes start falling, take a compass bearing for reference as visibility worsens. In a blizzard, backtrack if you can, or continue following bearings to move forward.