First off, my 'Net friend, Lisa Ross, has already put together a great set of tips/how-tos on matters related to anal sex: Tips for Tops on Anal Sex. You should go read it.

Also, I did a bit of research on the Web and found tons of info on enemas. Here is a selection of the more informative ones:

• Sexuality.org's "Your First Enema"
  If the above link is broken, try this copy.
• Taking a Comfortable Enema©
  If the above link is broken, try this copy.

The most important thing I learned was to receive the enema laying down & not putting any strain on my stomach muscles, to allow the water to work its way in more deeply. One Web page recommended a position similar to the first-aid "recovery" position: to lay on my side/stomach, one knee drawn up into a 90' angle.

In that position, the stomach is totally relaxed and it takes very little effort to receive & hold the enema charge. Previously, I usually sit on the toilet, which is not good, since a. the muscles used in sitting upright interferes with receiving & holding, and b. the anus is at a lower position than the rest of the colon.

I spread a couple of nice thick fluffy towels on the bathroom floor. I like to be next to the toilet bowl, just in case . Extra touches of decadence would be heated towels (tumble them in the dryer with a fabric softener sheet or two for a while), extra heated towels to use as "blankets", scented candles, dim lighting, rose petals scattered all around... Oops, forget that last one; got carried away; it's just an enema ferPetessake! ). OTOH, it may make for an extra-sexy experience if your lover is giving you the enema!

I can get myself squeaky clean inside after two or three fill/discharge "cycles", holding each charge for about five minutes, ending with a "partial fill" cycle, which I hold while I go do something else, perhaps put on make-up. Half an hour later, another discharge/fill/discharge cycle and I am clean enough to be eaten down there!