#1 thing: become aware of how your gut works. Start paying attention to when things tend to move around, because it generally is kind of periodic. Use this awareness to figure out times to play and times to avoid.
When you want to play, check things out first. Use a finger or a toy to see if you're clean. If you are, then nothing to worry about. If not then try and go. If it still doesn't work than try other ways to clean out.
Elnoa recommends against plain water, but for the most part plain water is better than most mixes out there. There is some evidence that a PEG-something solution (a similar compound to what's in Miralax) is easiest on the colon, but water is better than soap, commercial chemical mixes, vinegar or lemon juice (yes, believe it or not, I've seen these recommended somewhere... they're potentially dangerous and can cause colitis). If you have the time a pinch of salt and soda (don't use too much!) will make the water more "butt friendly", but I'd only use one of these three solutions (PEG (never tried), water, or salt+soda). Only use as much as you need to fill the "play area" - don't go squirting a quart up there if you don't have to.
Another alternative is glycerin suppositories. I've heard from some medical professionals on sex boards that they're gentle enough to be used a couple of times a month or so and they don't require messing with water. Just don't use bisacodyl.