We started out using some common sense the first time we played butt plugs starting out with the small plugs and working our way up, and even years later neither of us have even thought of attempting the large ones you can buy. We seem to have stopped at the medium sized plug and that seem to fulfill our needs perfectly. It makes me feel full enough when I'm masturbated with the plug in place and open her up enough so I can get inside and fuck her ass.
What I have found is the ribbed plugs are slightly better for me when I'm being masturbated and the plug is played with, all the same I love the feeling of the smooth plug. There also something special when the plug plops into place and the thickest part enters your asshole and I love the feeling when it is taken out.
You have to be careful not to over use them from what I have read and we tend to limit their use. We take the approach that in moderation they are great fun, but neither of us wants to spoil the fun by stretching the muscles to a point when that certain tightness that I love when I fuck my partner in her ass is lost. The other thing we make sure of is we use plenty of lube, which make it better for the both us, after all the idea is to have fun.
Earlier today we took a trip to the local sex shop where we purchased a Tantus Bend Over Beginner Harness Dildo Kit so it will be out with the butt plugs over the weekend to open me up. I can't wait to feel how it feels to be fucked in the ass. From all I have read and heard it is going to be great, but I know we will take our time getting there.