I currently use either Mr Super Clear or Gaianotes, usually matte. When I sprayed acrylics, I used Vallejo varnish.

@Akaru88 I've used Testors Dullcote, but I always seem to waste too much of it; one can usually covers roughly 2 1/144 scale kits. I've had more success with Vallejo Matt Varnish in an airbrush, but it does require said airbrush. A shop near me recently started stocking Mr Hobby Clear Matte, so I might give that a whirl if I ever feel like spending the money.