How can I fry leftover mashed potato? Today I need some tips on reusing mash potato. And not the "don't do it" tip. I mean, how can I fry it?

So far I've pretty much limited my reuse of mash potato to fried pattie-type things, but I found this doesn't work so great with every mash potato I come across. Sometimes it's either too dry or too wet. How can I fix that?

I usually sneak some cheese into mine when I make it, but the people I live with make it very plain, and anything from crumbly to watery. The crumbly ones to me have been essentially useless because I don't know what I can do to it after it's been made to make it okay again. For instance could I later add cheese or even an egg or some other bonding agent so it stays together in the pan long enough to seal and survive being manipulated a little with an egg-flip?

On the other hand, when it's really watery, what can I do about that? The limits of my knowledge of thickeners available in your average kitchen are flour and gravox, but I haven't been game enough to mix either with mash potato and fry it since one time when I had dry mash potato and some leftover gravy I just mixed it all together and fried it thinking, surely it will all just mix together and be wonderful. But it was terrible. Not to mention it kept almost burning because the gravy seems to seal a lot quicker.

Edit. I am indeed after any suggestion, not limited to frying. @Jefromi decided to edit my question to make it sound like I'm only interested in how to fry it.
For one half of your problem: You can add a little bit of milk to dry mashed potato to make it moister, if it's too dry to use in a given application. The other half is a little more difficult, but instant mashed potato can be used to sort of "soak up" some of the moisture to make it drier.