I second the suggestion of being more patient with douching. Don't go straight from douching to anal play; if you just hang out for a little while after douching then the leftover water will come out eventually. Wait for that to happen before playing.
Stretches can help you relax too. Pigeon pose is my favorite when it comes to prepping for anal sex.
pay attention to your muscular responses when doing anal play and you'll realize you can have control over them if you can breathe and consciously relax. Notice when you're constricting your sphincter or, more importantly, tensing your abdomen, and try to breathe through it to overcome any involuntary muscle responses.
Know the routines of your peristalsis, there are times of day when your body will be more likely to anticipate a bowel movement than other times. If you're a shit-in-the-morning guy, wait till the afternoon to douche and play. Alternatively, if you're a shit-after-dinner guy, maybe reserve your early morning for anal play.
No matter what, we have to cooperate with our body's needs when it comes to anal and even the most experienced bottom will have times that just aren't the right time. Don't be discouraged.