Catt,
This is an awesome idea. The more ideas we can come up with, the more ideas we'll have when we read someone elses.
Styrene to Wood:
Use a file and scrape across the styrene the way you want the grain to go, then use a black ink to rub onto the styrene. Use a cloth to wipe the stain off the raised portions of the styrene, leaving the ink in the "grooves", then paint over top with your wood paint. Works great, more knots can be made with an x-acto blade. The ink will form shadows in the grooves.
Cheap sacks of sugar or flour or cement:
I roll playdoh into a long string and cut it into little pieces. Using a fork I flatten the ends a bit, and use the end of an eraser to shape the bag to look worn or full. Let dry overnight or until hard and paint.
Details for machinery:
If you are into building model ships, cars, and airplanes in any scale, never throw them away. Parts and pieces cut or as is from numerous things in these kits are perfect for machinery or airtanks, or other details.
Cardstock model warping:
I have found that by first smearing model glue all over the piece of cardstock prevents it from warping if you leave 1/8 " on either end of the piece and put books ends on that 1/8" strip...works great and allows minimal warpage even by humidity.
I'll include more tricks and tips I have used once I write them all down to remember them.