are these the flexible type flares or the really hard ones. I am not sure which you are refering to. But if they are flexible you want to get Flex bumper paint you can get that in gloss, flat and pretty much any color you want. but the prep work is what is needed most. The red scotch brite pad is great for this as suggested by ems 34 20. but make sure you completely dull the paint. then wipe it in alcohol. this will show you were you missed with the scotchbrite pad. then when you think you are done scotch it one more time. then you can paint directly with the flex bumper paint or you can get a flex primer but not really needed.
If it is the hard flares then just scotch brite and any paint will do. preferably a eurothane of some type though. Still has a bit more flex then straight enamel.
I have done this numerous times having owned a body shop and building custom cars, painting boats, etc.
it is all in the prep.