Shredder, I love the ways you've done this with you wife. I'm jealous. I've given my wife a few enemas, but she doesn't enjoy them nearly as much as I do, and lately she's declining to even give me enemas--much less masturbate me during one, or peg me while I'm retaining.
I'm like afternine. I like doing myself in the tub with a dildo while holding a medium sized enema. I'm sure it would be a lot hotter with a guy fucking me, or a gal, (preferably my wife, but probably not going to ever happen), doing me with strap on while I'm holding 2 or 3 quarts. I fantasize about things like that, but doing it solo is still a huge thrill.
I do myself in the bathtub or shower with a 2-foot double dong. I take several large enemas first to clean out like afternine suggests. Then I pour a generous amount of mineral oil into the tub and side up and down the tub on my back until I'm bouncing off the front and back of the tub like an air hockey puck. Next I weight down one end of the dildo near the tub drain with a dumbell. The other end of the dildo I lube thickly with Vaseline or lard.
The final step is to give myself a medium sized enema, and impale myself on the dido. Soon I can't help sliding up and down the tub--sometimes at a furious pace. The sensation of sliding 12 inches up and down the stationary dildo with 2 or 3 quarts of water inside me is hard to describe, other than to say it never fails to make me cum, and it's well worth every bit of the trouble of the extended preparation time and clean up.
Like Shredder says, I tend to have a big leakage, a partial expulsion actually, when I cum. It would be pretty messy for me to do this with another person unless we could find a really big shower we could stretch out in. The next best setting would be a very private backyard patio or deck that could be hosed off into the grass after we hose off ourselves. Also, both partners have to really enjoy enemas with each other and have a big tolerance for being messy together, at least if I'm involved.