You should be able to get a blocking board from Larson or United. Otherwise, check with stitchery shops in your area. A lot of them carry blocking boards for stitchers who block their own pieces.
-You can block the piece either face up or face down, but... keep in mind that if you block it face down it will squish and flatten the stitches - and the texture of the stitches are part of what makes needlepoint have the look it has.
-You can block the piece more than once, just wait for it to dry (keep reading) between blockings. Each time you block the piece, it gets easier, and the fibers soften up and become more elastic.
-The best way to soften up the fibers so they can be blocked is to use to a steam iron 1/4" ABOVE the piece. Don't let the iron touch the piece since it can flatten the stitches or create shiny spots!
-Pin one side first, then steam. Then pin the next side and steam again. Work your way around until you have it all pinned. (Pins/nails should be placed about every 1/4" to 1/2".) If it isn't perfectly straight, let it completely dry (at least 2 hours), pull the pins out and block again.
-Let the piece dray laying horizontally - don't stand up the board while it dries.
-You do have to be careful about not getting the piece too wet so that the colors don't run (although MOST shouldn't, some do!). The steaming technique should keep the piece from getting too wet.
(In case anyone wants to know, I got this information from "The Needlework Doctor" by Mary Kay Davis. I have done a few needlepoints that were REALLY skewed - one was stitched back in 1975 and had never been blocked - it was almost warped 45 degrees, but it came out straight in the end!)
Why the pieces warp:
If they use Continental or half-cross stitches, they will warp the piece. If they use a basketweave stitch it won't warp the piece. The basketweave stitch will stableize the piece even if there are other types in stitching involved. If you have a new stitcher who brings in many warped pieces, you might suggest that they try to use a basketweave stitch to help keep the piece straight!