Also, you need a National Forest Recreation Pass to be able to park at the climber's bivouac.
The south face of St. Helens is the most popular ski route of the volcano, since it follows the same slope as the usual ascent. Make sure to take your time at the summit to enjoy the spectacular view of the caldera and Rainier to the north, before dropping into over 2,000' of open, consistent corn. The descent is pretty straight-forward, no crevasse danger, no really hidden obstacles or traps, just beware of sudden slope changes and rocky areas (good potential for localized avalanche triggers). Just ski down to the hiking trail and follow that out the same way you came up.