If you don't have the nifty vacuum brush already mentioned, a good lint roller is priceless for these things. The kind you get at the dry cleaners. If you get a cheapy one at a dollar store, they are a pain in the neck, but the good kind work great and last well. I don't know what they cost the first time, but around here the replacement rolls are five dollars each. We use two or three of them a year.
If you already have the glass on and don't want to remove it, but need to get a crumb out, try this. Get a piece of removable scotch tape and a little strip of matboard (about 3/8 inch wide and between three or four inches long). Raise up the edge of the glass, slip the tape in with the sticky side down so it's above the crumb. Press it down onto the surface with the matboard strip. Pull the mat strip out, pull the tape off, and the crumb should come with it.
If you have a lint roller or vacuum brush, you shouldn't have too many little crumbs left once the glass is on. If somebody marches onto this thread and shares a better way, go for it, but this is a fast, handy way that works for me.
One more thing. Some people use a Jiffy steamer to get scuff marks and lines off of suede mat boards. I just store them carefully so they don't get the marks in the first place. Used to work at a place with a steamer, though, and it worked well when they used it. If you ever do that, be sure to give it plenty of time to dry completely out before cutting it.