Judy, whenever I am painting something that requires a thin edge or a line, I paint it in with a fairly large brush ("large" being relative to the size of the edge and the size of the painting)...larger than instinct would tell you, anyway, and then my secret is to *adjust the surroundings*.
It doesn't matter if I'm painting wet into wet or wet over dry. I don't worry about getting it exact at the first strokes because I will need both soft and hard edges, anyway, which are easiest to produce when you work on the adjoining areas concurrently.
I don't know if that's the best explanation ever or makes much visual sense, so let me try to detail it out.
If I were painting a bowl, I would first block all of the general shapes in, not trying to be super accurate. Then I'd swear up and down that I would work on it again the next day, but in reality I probably wouldn't get to it again for another week and so it would be dry, which is fine. When I go to correct the details (after I've washed the hardened paint out of my brushes, of course), I would take my filbert (I use filberts for darn near everything) and make a *swooshing* motion along the edge of the bowl area, holding my brush with a stiff wrist so that my whole arm is moving. I would try to get it as close as I could, of course, but in all likelihood it would be too thick or perhaps the block-in would be showing because it wasn't all that accurate.
Then I would take the adjacent color of the table or the inside of the bowl or what have you (or some color close to it--when necessary, I can mix in a little something else into the wet paint on the canvas to get an exact match) and I would "clean up" the edge (using the adjacent colors) so that it is the proper thickness/shape. At that point, it might be a fairly hard edge and I would then wipe my brush off and lightly blend just where I want the soft edges to be (blending just the edge where the objects meet, not the whole brushstroke) if the soft edges haven't occurred naturally with the wet paint in the first place (I don't do much blending really, if hardly at all).
The joy of oils is that you can go over an area again...and again...and again...until you have it as you'd like. If you end up building thicker paint on the canvas than you'd like and it's getting difficult to control, it's a simple matter of scraping what you have off gently with a palette knife and continuing onward.
I don't know if you were able to follow along with all that and visualize it. I was just painting a sugar bowl this week (you can see it in the February paint-a-long at the top of the forum) and the handles were giving me the fits, but I finally got them...generally accurate.