Lorna,
As you well know, I've seen many of your photos. While you may not have had any formal training, you have a natural 'eye' for composition; something that many strive for but few achieve. When you take a color picture that you think might look good in b/w, think 'black and white' and try to visualize the scene without color; that will make things much easier. Years ago (and still today to some extent) photog's would shoot with panchromatic b/w film which is insensitive to reds, so they had to visualize how the finished shot would turn out (reds appear black). Grey scale range is another important factor to take into account, that is, if you expose for shadow detail, what will happen to the highlites - will they be washed out? Some of this can be corrected in the darkroom, but if the details are missing from the original, there's no way to recover them in the final print (short of redeveloping; but that's a whole other subject). Many people use PhotoShop, a very good editing program, but I use Microsoft Image Composer and/or Kodak "Picture It '99"; both free and included with Windows 98 and the Kodak DC-240 camera (respectively) I used to have. Picture It is somewhat limited but does an amazingly good job of removing scratches and dust specks from old photos.