Cutting in well is a skill, not a trick, but it comes down to three things: the right brush, the right load, and a steady hand.
Get yourself a quality angled sash brush — a 2-inch is the sweet spot for most cutting in. The angle lets the bristles bunch into a fine working edge. Cheap brushes shed and splay; they'll never give you a clean line. Spend the money once.
Loading matters. Dip the bottom third of the bristles in, then tap (don't wipe) both sides against the inside of the tin. You want the brush charged but not dripping. Too little paint and you'll drag; too much and it floods the line.
Now the technique: don't start right on the edge. Lay the brush down a few millimetres *away* from where you want the line, spread a little paint, then gently push the bristles up into the corner or edge and draw along it in one confident pass. Keep your eye on the leading edge of the bristles, not the ferrule. Slow and steady beats fast and shaky every time.
The biggest mistake is letting the cut-in dry before you roll. Always cut in and roll the same wall section while the edge is still wet, so the two blend with no visible band. Work in patches — cut a metre, roll it, move on.
For forgiving edges, a matt emulsion like Little Greene Intelligent Matt or Farrow & Ball Estate Emulsion hides minor wobbles better than a sheen finish, which catches the light and shows every overlap. Trim is where it shows most — take extra care cutting in against woodwork.
FrogTape is fine for getting started or for tricky junctions like a ceiling line against a dark wall, but lean on it as a crutch and your freehand never improves. Burnish the tape edge down hard with a clean finger, and pull it off while the paint's still slightly tacky to avoid peeling.
Practise on a hidden wall first. Within a couple of rooms your hand steadies and you'll wonder why you ever reached for the tape.