DISPLAY SEAL -
Once you've finished the miniature, it's best to seal it with a coat of spray sealant, which will help keep the paint protected during gaming use. I use Testor's "Dull Coate", which can be found at any hobby store that sells Testor's paints, or Krylon Crystal Clear, an artist's sealer available at art supply stores. For the amount you get, the Testor's is more expensive than other brands, but I like its look the best. Dull Coate puts on a very even matte coat which brings out the full details on your paint job..
If Testor's is too expensive for you, other manufacturers (GW, The Armory, etc.) make spray coats. Just be sure you use something to coat your miniatures, or you'll be pretty upset when the paint begins to rub off from handling, or chips begin appearing after each gaming session. Avoid hardware store sprays or brush sealants (Varathane, etc.): they 're just too thick to use on figures.
For the best results, use the same spraying procedure for the sealer as you did for primering the mini: spray multiple, light coats.
As a finishing touch, I also give any metallic or reflective surfaces (armor, weapons, gems) a coat of brush-on gloss sealant; Ral Partha's "Clear Sealer", thinned with a drop of water, works great. This helps make those surfaces appear hard and shiny, like they should! Also, if you have any shiny area that the matte spray didn't adequately cover, you can use a brush on matte varnish to cover these (I recommend Vallejo Matte Varnish) - again, just don't brush any varnish on too thicker.
GAMING SEAL (aka 'Bulletcoating') -
For your most heavily-used minis (such as wargaming figures), you may want to spray them with a few, thin coats of a gloss sealant (Testor's, etc.) followed by a few coats of matte, or just use brush-on matte sealer straight out of the bottle. Take note that the gloss can obscure blending work and give the mini a 'plastic coated' look. The matte spray helps combat this, but even so I wouldn't recommend this procedure for prized miniatures you're painting for display, as subtle blending and some details can be lost (like the time I once spent all day blending gray and black paint for a drow's flesh, only to see all that work disappear with a coat of gloss sealer)..