I use a thorough strip and clean after every airbrushing session, theres no substitute for thorough, no special cleaners, just the thinning agent appropriate for the paint i'm spraying.
A quick overview based on Enamels
Materials:
Cotton buds
Paper towel
2 containers, (I use a tin and a plastic pot)
Turps
Water
Procedure
Wipe bowl clean with paper towel
Pour in a little drop of Turps, backflush then spray into container one, tip out then wipe bowl, (do this again another once or twice until the turps in the bowl is near enough clean), twice is usually enough
Strip brush as follows
back of body, screw off and place aside, loosen needle chuck and draw needle back a couple of mm
Take off cap
nozzle cap
Nozzle
Place all in container two with enough turps to cover, (you don't need much)
Loosen needle chuck and push needle out until you can grip and draw it out from the front
Use cotton bud, (one end dipped in turps) to clean bottom of bowl / needle hole area and tip where nozzle screws in
dry up residue with dry end
Use paper towel dampened with a little turps to wipe needle whilst gently rotating it along it's length, dry with towel.
replace needle from front drawing it back so needle is drawn into body, tighten chuck.
Remove nozzle from turps bath, prod a brush through to ensure it's clear, blow through, refit
remove nozzle cap and air cap, clean with cotton bud, (as above), replace
Loosen needle chuck and push needle into position and tighten, replace rear of body.
Part fill cup with water and spray to check pattern and correct seating of needle, (better to find out now than after you've loaded it with paint the following day)
I do this over a hard surface and take care with the needle
It sounds lengthy but provided everythings to hand it's very fast indeed and the brush will be spotless. Acrylics is the same with Isopropyl Alcohol, (you could use an ammonia solution)
Note, this is after a spraying SESSION, not after every colour, a quick flush through between colours is adequate.