When I was doing this, I just painted the lighter color first, let it dry, then put painter's tape on the painted edge and added my new color. But I bet @Peace Painting has a more professional answer for you.
The tape method is a good idea, especially with the new sealing tapes like Frogtape. Get the one for delicate surfaces if you are going over a freshly painted area.
Pros rarely use tape to get a straight line because they have spent years 'finding their line'.
This also depends on the walls texture, smooth wall are far easier to use tape on...with practice "cutting in" without tape can work better and faster. my preferred brush for this is a 2" angled sash brush..Purdy.
to any corner there is a natural line if you run a corner of a putty knife lightly down the corner it will give a place for your paint to run to and you can push it in the corner with a good purdy brush another thing that helps with paint bleed under tape is to run a putty knife swiftly across the tape what that does is activates the wax in the tape by heating it up.
All good tips,I like to use a clean rag to rub over the tape.Its soft and less chance of damaging the tape. As a pro i free hand my lines.But something that does help is this.With a clean rag and a putty knife, i pull it down the areas that may need a lil attention. The thing here is depending on the angle of the putty knife it moves your line a little.