My recommendations:

Warm water enema (not chemical...), flushing a couple of times until water runs "clean".  You can get a hot water bottle type kit at pharmacy or get disposable and empty out chemicals and refill with warm water.  Chemicals are not good for you, and are more used when there are bowel movement issues.  I recommend doing this 2-3 hours before play or earlier in day to give colon chance to recover (too much stimulation at once when new can limit stamina/tolerance later...) PLUS sometimes water continues to work itself back down even an hour after...  

I honestly find that enemas are rarely needed when you a.) eat a good diet (fiber rich w/ fruits and veggies) and b.) know your bodies rhythms.  (ie: usually after you eat, your body is stimulated to go to the bathroom about 20-30 minutes later...  Don't eat then try anal play. lol)

Do NOT use numbing agents.  Any receiver should be able to feel what is going on as if it hurts, your body is saying "STOP or you are going to DAMAGE me!"  Anal sex, when done properly with lube, warm up, good communication and not rushing anything should not hurt!

Thicker lubes are better for coating/protecting delicate tissue.  Try Astroglide gel, but ultimately this is something that can vary from person to person on what they like.  

Standard warm up should include:

*stimulation on outside
*teasing opening
*slow insertion/teasing w anus-small toy or 1 finger
*graduation to medium sized toy/2 fingers
**I recommend receiver being comfy with 3 fingers before moving to anal intercourse

Do NOT fuck an ass like you fuck a pussy.  It's not the same.  Doesn't feel the same, doesn't respond the same... Start slow and have her tell you how she wants it.

How fast each of the above progresses usually depends on the experience/frequency of play of the bottom. More experienced may not need all of the steps above for warm up.

I recommend wearing a latex or polyurethane glove with digital penetration to avoid small tears from nails or hang nails that can later prevent progressing or decrease longevity of play.  

Allow receiver the ability to stop play whenever their body tells them, and to not push when first starting out...  If they want to push it later, after they get to know what responses are what, that is up to them.  

Hope that helps.

xoM

read what Megan wrote but to take this experience as painless as possible?  unless you are 7" long and slender your SO wont have a problem but if you are 8-10" and thick? forget it.. we gals can only stretch so far .. before we rip ..