I used the open grid method (probably the strongest and easiest) using 1x4's for the framing and 2x4's for the legs. The area that is designated flat terrain is covered with 1/2" BC graded plywood using plenty of screws. In fact, use screws for fasteners in the entire construction. (Save money and use drywall screws w/pilot holes.)
The joists are set at 16" centers. Legs are supported with 1x2 diagonal bracing.
Yes, I did have to stand, sit, and lay on the benchwork during different phases of layout building...and still do. I'm 190lbs and it didn't flinch. While this design is very strong, you may not have a need for such. As an example, if you have a narrow, around the wall, type of layout then you wouldn't need that much strength. Also, if you plan on moving it around you would want a lighter construction method.
L-girder is another very popular and strong method, but requires a little bit more time in construction. One advantage it has that I like: you can easily shape the edge of the benchwork with flowing curves if that is desired.
One thing to keep in mind is the underside; it will get almost as much attention as the top. You will have to make sure that your bracing will not interfere with under-the-top turnout motors (Tortoise) and any wiring of lights, detectors, Turntables, and etc. With mine, I simply framed these areas a little different if I thought ahead of time. There were times that plans were changed and I had to move a joist or notch it for an installation. This is not a big task and is easy enough to do.
Hopefully, these photos of my first half will give you some idea. Click the link at bottom for more photos.