Here is my $.02...
Think about the selfish, lazy and small-minded audience you are attempting to address. I suggest that you totally forget about writing the "nicey-nicey" memo... And forget about writing the "in your face" memo... those memos won't work!!!
You gotta get down to brass tacks with these people!!!! You need to address the issue privately...one-on-one with each individual who is specficially abusing this privelege and tell that person the low-down plain and simple ... that use of the kitchen is a PRIVELEGE and that they must clean up after their own selves or they (and everyone else who works there) will lose the kitchen privelege.
Unfortunately, it's a shame, but sometimes we must stoop down and talk to adults like children.
If that doesn't work, another thing you could do is bring in a bunch of those plastic bags from the grocery store. Take the dirty dishes out of the sink and put them in a bag and place the bag on the guilty party's desk with a note asking them to please clean up after themselves at the office or take their dishes home to clean.

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Sometimes you just gotta get right down to the nitty gritty to get your point across.

This tactic will work...I PROMISE!