Several good ideas have been expressed... let me share another. To make trenches, I assume you are doing a thick base to show level ground as well. Here is the technique I use - quite easy, and very inexpensive. At the florist shops they sell a thick green foam that can easily be cut and shaped with a finger. I mount this to my wooden based, and then begin to shape the trenches, shell holes, puddles - whatever. Once the contours are correct, I then "paint" a solution of plaster of paris over the entire piece, which gives it hardness and texture. Then you can begin to treat the area with whatever ground cover you so desire. Like Faust, I believe that varnish makes an excellent gloss, but if I want to show standing water puddles amidst the footprints and holes, I use POUR-ON by EnviroTex. It provides excellent wet gloss, and also good "water" depth. As a side note, to make footprints in mud (snow, dirt, etc.), i have sawed off two "feet" from a figure and attached them to long strands of stiff wire. I can then use these feet to make impressions whereever desired... even in difficult areas to reach, simply by bending the wires as needed.