Be careful of tires...they work great, but can be very expensive to dispose of if you ever have to.
Carpet works well when stretched, but it will hold paint, sand, and dirt over time. Which sucks if you are hiding behind it and someone is pelting the other side with paint. This makes all that dried crusty dirt and old paint fly around and choke you out.
Pallets, spools, and other wooden and plastic sheeting works best because it is the most versitile. Watch for old houses being torn down, you can usually save fencing and wall material without much effort and at no cost. Old doors work very well also.
We also used to stake in 4 pieces of tubing or pvc into the ground, and stack downed trees and branches between them, creating the same effect as a pallet. This was cheap and when one wore down we just grabbed more downed limbs (or cut some down to clear other parts of the field.)