Are the trees in foam or plaster? If they are in foam, I drill an undersized hole for the trunk, put a dab of Aleen's Tacky Glue on the trunk and jam it down in the hole. The combination of the foam and Tacky Glue will usually hold the tree in the right position. I like Aleen's because it's more aggressive than white glue and sets up faster, so the tree doen't have as much time to fall over. If you get one that won't stay up straight, use a tooth pick as a prop until the glue is dry.
If it's plaster, drill a hole that's about the same size as the tree trunk. I then slather the bottom of the trunk with Elmer's Carpenter's glue, which is much stonger than white glue in plaster. Put the tree in the hole, hold it for a minute, and see if it's still straight. If not, use two or three toothpicks in the hole to adjust the position. Put them as deep as they will go and add a little more glue. After the glue has dried, cut off the toothpicks at ground level with some nippers. Sprinkle a little ground foam at the base and no one will be the wiser.