Doing a 50/50 on a snowboard is a crowd pleaser in competitions and just something fun to do while riding down the mountain on your snowboard.

[edit] Steps

Look for a box or rail where you can ride straight on to the feature without needed to ollie very much (or at all). Make sure it's long enough to perform your stunt.
Ride down to the rail with cautious speed. Bend your knees to absorb the impact of small bumps or cracks.
Ollie up early, if necessary. Be sure to jump high enough to get up and over the rail.
Straighten your legs to land the snowboard on the rail. Land your snowboard flat on top of the rail. Don't lean on your edges as if it were a hill.
Land the board lightly and don't push your weight down on the board yet.
Bend your legs so your weight is pushing down on to your snowboard. Your weight should be fairly evenly distributed between your feet.
Jump lightly off of the rail or other object with straightened legs then bend your knees as you land to absorb the impact.

[edit] Tips

There are many variations of this trick, so just be creative and try them.
There are many objects that you could do this off of.
When on the rail, look at the end of it. It will help keep your balance and keep you focused.

Be careful to watch out for surfaces that could damage your board.
Don't try this until you can do an ollie.
Snowboarding is an inherently dangerous activity that can result in serious injury or death. Seek proper training and equipment before attempting this activity.