Testor's dullcote has always served me well (after a gloss coat of Future for protection).
I just recently started airbrushing my dullcote using a small bottle of "matt medium" (should probably be matte). I got sick of having to buy a new can of Testor's every other week. Airbrushing is also nice cause you can do it inside, avoiding all the climate related problems of varnishing.