Answers

Typically flat paint is not cleanable, unless your flat paint is Sherwin Williams Duration or Emerald in which case you can use some warm water and a sponge.

If it’s not one of those two products you really don’t have many good options.  You can try cleaning it with warm water and a soft sponge, but be very careful not to remove the paint from the wall.  You can also try touching up the dirty areas by feathering the paint in, and as a last resort you can repaint the wall.

Flat paint typically isn’t the most live able finish, but it does provide you with the richest appearance.  Sherwin Williams Duration or Emerald gives you the best of both worlds.  Duration is my companies go to interior wall paint, I highly recommend it.

I FIND THAT USING A MR. CLEAN ERASER PAD WORKS GOOD!

I use windex very lightly with a soft cloth

Philip

As others have said, Mr. Clean Magic Erasers work quite well without compromising the integrity of the wall finish.

Magic erasers by Mr. Clean tend to work really well without causing any paint discoloration. 

Mr. Clean Eraser Pad

I would have to agree with the Mr. Clean pads. The best way would be to paint over it only if you have the same flat paint...but I know sometimes it gets lost or dries out. So the eraser pad should do the job.

I would echo what Steven says. Generally, flat paint is not washable. If you look at the cleaning instrctions, it will talk about being "scrubbable". This means you have to scrub a layer of paint off to get the dirt out. That is why the Magic Eraser works for a while. Ultimately, if you can simply use a degreaser and a rag, do it. If not get ready to paint with a "washable" flat or matte finish. Benjamin Moore also makes a very washable matte finish called Scuff-X.