I'd seen a thread on this topic some time back - what seems to be
working for me, an avid non-painter, while painting walls at the
Those foam-rubber-on-a-stick cheap paintbrush-shaped applicator things.
I never thought they amounted to much, but they're perfect for this job
(they come in many widths; get a fairly wide one). Basically, put some
paint in a shallow tray. Dip the edge of one side of the applicator in
to get a small amount of paint along the entire edge (we're not looking
for volume coverage here). Lay the paint-laden edge just under the
desired spot and nudge it up carefully. Pull down and feather out paint
so no 'seams' will show under the main coat.
Note I'm not pulling the paint along the line, and I'm dipping with
every application - this is not the place to skimp on paint. It all
seems pretty obvious writing it down, but the main thing is the neat,
straight edge of the foam-rubber applicator. Takes a while, but I'm not
having to go back over mistakes with the other color, then doing it
again with the original, etc.
BTW, I find it helps to be standing on a sturdy platform - lets me get
close to my work, and I don't sweat teetering on a ladder while I'm
concentrating on being steady.