Supporting your yard is not much different than supporting the rest of your railroad. There are a few things you may want to consider up front however. If you have a track plan for the yard before you start building, you can incorporate a few nice features during construction that can make future steps easier. Even a rough plan can be a big help at this stage.
Yards have lots of track and turnouts. If you are using under-the-table turnout motors, you'll need a place to mount them. Murphy's law dictates that there will be a support in the way when you get the track positioned just where you want it. If you know where the ladder(s) will be placed, you can adjust the spacing of benchwork supports appropriately to fit. Moving a support an inch or two won't have any dramatic effect on the stability of the railroad, but it can save hours of fuss when it comes time to power those turnouts.
The same is true for turntable pits. If your yard will have a turntable, transfer table, or any other recessed feature, plan it's location and build around it.
Most people think level when they think yard. In some cases this is true. You don't want cars rolling back out of the yard on their own. Some grades can actually be an asset in a yard however. By lowering classification tracks into a "bowl" you can help keep the cars in place. More extreme is elevating one end of one yard ladder tall enough to create a hump and allow gravity to switch the cars for you.
A good hump should have a well-defined curve at the top to aid in uncoupling, and a grade that starts steep and gradually levels out through the ladder. The exact percent of grades will depend on how much space you have in the yard and how well your cars roll. Too much speed is as bad as too little. A slight raise in the other end of the yard will keep the cars from rolling too far. Keep this grade minimal however to prevent cars from rolling too far back toward the hump and impacting track capacity.