Laser-grade plywood or MDF
Sprayable clear wood sealer
Step 1: Apply painters tape
The lower-tack the tape, the less likely it is to pull up the grain of the wood when removed.
Applying tape to both sides will prevent the appearance of smoke marks on both sides, but may not be necessary if only one side of the final piece will be visible.
Do not use plastic-backed tapes. Most will melt providing poor protection and possibly marking the piece, and vinyl tapes will emit chlorine fumes, which are poisonous and can damage the machine.
Step 2: Etch and cut
If you are intending to apply color to the work, still perform the cuts, but only etch the fields in which you do not wish to apply color.