Bob's pretty much said it all.
I use a framed piece of 1/4" glass.A door off an old cabinet that I found someplace thats like 3' long and have 3 strips of roll sanding board paper on.
When you true up the Durablocks on the 80,you'll SEE instantly where the high spots are cause they will turn a dark black and be rougher than the rest.
Apply enough pressure to make the board go and cut with steady,even strokes. The "idea" is to let the board cut itself down with as little downforce as necessary cause if you go pushing too hard on the rubber,it will distort it and ruin the process. Wood can take more pressure but you can still distort it as well. I've even taken my wood long boad and used it on top of the Durablocks as a helper to keep things true.