I just take the big ones that are getting old (around 2 yrs) and put them in a closed container like rubbermaid. I hook up a CO2 tank with a hose and gas them. Then it's into the freezer. Some of my really big girls can pound them down. My son inlaw has a red tail boa so he gets a few once in a while.
I feed live, all my surplus rats go into the freezer too. I keep them in groups of 1.3. When I get to many I cut back by removing the males for a while.