What is the best way to mask the door and trunk jams to prevent those unsightly paint lines? I'm looking for options.
The best way is to paint the jambs with the outside.
If that is not a choice for you, you can use 3M foam tape.
They used to call it D.A.R.T. tape, I think they call it something else now.
It is a foam tape in a couple sizes that has adhesive on one side. It is positioned in the door jamb and then the door is closed. It gives a "soft" edge to the paint instead of a hard line. It ain't perfect but better than a hard line or nothing at all.
3M also has a soft edge type tape they came out with in the last two years. It is a plastic tape that only has adhesive in the middle. I have used this in multiple layers, peel one off after sealer, another after base, the last is for the clear. This is a little better than the foam but you gotta open the door between coats.
Sorry I don't have the part numbers here now, I'm not at the shop.