Well when it comes to wearing a butt plug all day long, it can be a bit of a stretch to expect it to always be spotless when you remove it. Timing can go a long way - try to use the bathroom early in the day before inserting your plug. The occasional enema can also be quite helpful, although taking an enema too often can cause side-effects for some folks (I much prefer the less-extensive and quicker anal douche).
I make sure to have access to a zip-lock baggie just in case my plug happens to be rather messy and aren't able to wash it in a private sink (I can't imagine "Don't mind me, just washing my laundry..." going over very well with co-workers ;P)
One issue about all-day plug wear that has caused problems for me in the past has been excess lube showing up under my clothing. One solution to this that my Mistress particularly enjoys is to have me wear a snug fitting pair of panties with a maxi-pad under my plug. Getting used to it on a daily basis was a little humiliating, but it quickly proved itself to be a lifesaver.