I had a weekend job in a stencil store and used to do demos, so here's my 5c worth!
1). You absolutley dont want the paint to creep under the stencil, or you will bet a blotchy pattern rather than a crisp one. So get a can of aerosol adhesive, as used in art studios (spraymount in the uk). Give the reverse of the stencil a light spray, then it will hold in position without any edge lift.
2) You need a special stencil brush. They're not too expensive and make all the difference. eg
http://www.dickblick.com/products/blick-economy-white-bristle-stencil-brush/3) Put a small amount of your paint into a shallow tray. Dab the brush in, then work off most of it on a sheet of old newspaper. Now do a quick light repetative dabbing motion through the stencil. You dont aim to build up the colour with one stroke, but rather by keep going over the same bit.
4) I would then gently peel off the stencil while tacky, and only reposition it further down the tear when dry.
Hope this is of some help.
