Each series of supports are called bents. I would recommend at least two vertical beams, and two angled beams for your trestle, these will form your bent. At the top and bottom of the vertical beams you have what are called chords, or ledgers. The pieces that connect everything diagonally are called links.
Here is a nice site that gives you step by step instructions on how to build bents, and put them together in a trestle. Or you can get ideas and build your own. Here is a curved trestle I've built. I'm not done with it yet, but you get the idea.
I chose to hang the trestle before building the scenery. This proved to be both good and bad. As you can see I got plaster all over my bridge when I was adding scenery, but it comes off very easy with a toothbrush. The good side is that I could build a bridge a lot easier and cleaner looking this way. I made a stencil on paper that allowed me to mass produce the pieces in six hours.
I had to build some bridges across my doorways so I came up with a double platform truss. Build one level at a time, and combine the two. Then meet your track appropriately. These bridges are lift out to allow for walking. I'll have to load them later.
here is the one that I am building. 
