Some people have success with particle substrates like sand, but they are generally best left alone. I have never used or heard of clay substrates either. Non-particle substrates can't be ingested to cause blockage, and are much easier to keep sanitary.
Some good solid substrate choices are non-adhesive shelf liner and tiles. I use slate tiles and love them. Ceramic tiles also work as long as they have some texture to them. Your scaley little friend won't be able to get around if they are too slick! Hardware stores like lowes or Home Depot usually have some cheap tile (check the clearance rack) in stock and will cut them for you. Keep in mind that the interior dimensions of the cage will be slightly smaller than the exterior dimensions! Lay the tiles in the cage with no glue or grout so they are easy to get out! Tiles also help keep your beardies nails in check--No more scratched up hands!
To clean the tiles, you can remove them from the tank and scrub with soap and water (alternatively, rinse/rub off the poop and spray with a veterinary disinfectant such as F10SC). Then rinse well, dry, and put back in the cage! Rinsing well is very important if your soap or cleaner has a strong odor. You can also bake them in the oven to sanitize them. I would reccomend doing this for your first cleaning before putting them in the cage (the same goes for any pieces of wood that you use). Dry the tiles (especially slate), before baking so they don't crack.
Many people use and love the non-adhesive shelf liners. They are easy to cut to size and clean. I believe they are cheap as well! I would reccomend having two pieces that you can alternate for washing. Other probably have better advice on these as I have never used them. I'm not sure how others wash them.
If neither of the above works, paper towels or newspaper are two other cheap and easy to clean choices.
This was very long-winded, but I hope it helped! Good luck with whatever you choose
!
