an enema. If you buy the disposable Fleet type, make sure you pour out the saline solution or it will cause a laxative affect. There are also some great reusable products as well, Shower Shot hooks up to your shower,  or an enema bulb or even the good old fashioned hot water / enema bottle.  Just fill with tepid water and take your time repeatedly inner cleansing (water should run clear). Rushing this process will only trap water, which can come out later (deal breaker). Here is one of many informative sites and also written from the "straight" anal enthusiast point of view. Nothing is hotter than knowing your partner is clean and you can take your time and try many positions, without fear of them not coming to the session respectfully prepared.

For me it's a long process starting at least 3hrs prior, sometimes I start the night before.  The typical recommendations of a few cleanings 1hr prior doesn't work for me.  I think I have a fold/bone that traps things forcing me to make sure everything is clean up stream of it.  I use a large 4qt open top bottle and use all 4qts sometime more.   http://www.klystra.com/store/enema-bags/4qt-black-classic-rubber-enema-bag I don't hold but bounce up and down to agitate things.
I also use every inch of a very soft 12" dildo as a dip stick test.  It needs to be very soft to make it around the fold/bone without pinching anything. It also drains any water behind the fold.

It's proved pretty reliable and the dip stick test alleviates any worries of embarrassment.

Sorry if that's TMI.

I installed one of those hand held shower heads. You can switch from the attached head to the hand held by turning a lever. I take the shower head off leaving just the hose and turn the lever slightly to allow the water to flow thru the hose to my desired flow, not to strong. Flush until clean