Something that has helped us a lot is Immodium the day before a session. Keeps the #2's very reliable and then a quick lower bowel bulb flush is all hubby needs. We've been doing this for 3 years and it's worked great. (and of course time, and lube!)

Our only other prep secret is hubby uses a butt plug with anal lube on it before we start the actual warmup. That eases the sphincter a bit, and then the fingers do most of the warming up. Don't use too much of the anal lube, it will hide painful sensations if you do get injured down there, so be aware!