I'd first suggest finding a tailor whose skills are worthy of your confidence. You'll need to ask around for referrals. Also, before you have any work done on your favorite clothing, take something in that you'd like altered, but which wouldn't break your heart if it didn't turn out right. There's a reason why some ladies here call their tailors "wonderwoman." A good tailor is a gem.
I also have narrow shoulders, although I'm not quite as curvy as you. Here are some things I'd recommend:
1. Wear shoulder pads. I'm not talking about the atrocities from the 80's. There are nice, slim shoulder pads which can add structure to your garment and breadth to your shoulder line, which can be very flattering. This is by far the easiest solution.
2. Add a pleat or dart to the shoulder seam to take in a little width. This is the second easiest option and can add a nice design element.
3. Take off the sleeve and recut the armhole. I wouldn't do more than 1/2' to each side, since this can cause other fit problems. This is not easy, especially if there's a lining involved.
4. If there is a back seam to your garment, you can take in a little width there, however this can affect the fit of the back of the garment, and if done incorrectly, can also narrow the bust area, which may be a very bad thing in your case.
I hope that helps!
