Dear Paula,
First, DON'T blindly trust the one-over-focal-length rule, which I see has already been touted. It's an excellent rule of thumb but it is too generous with longer lenses, where you often need a faster speed. With wide-angles, on the other hand, you can often get away with longer speeds than the rule provides. I'd prefer 1/250 with a 135mm lens but wouldn't worry about 1/15 with a 21mm. Longer -- even much longer -- may be achievable: in the 1930s exposures of 1/5 second with 50mm were relatively common.
Second, accept the difference between 'acceptably' sharp and 'as sharp as possible'. Shoot a target -- a newspaper at 6 to 10 feet/2 to 3 metres is a good idea -- to see that even with a 35mm lens, while 1/30 may be acceptable, 1/60 will probably be better and 1/125 may be better still.
Third, have you been running? How much sleep have you had? Are you hungry? Excited? Angry? Any of these can make a difference of one shutter speed step or more.
Fourth, direct-vision cameras seem to allow longer hand-held speeds than reflexes -- so much so that it is worth putting a DV finder on a reflex. I don't believe it's mirror slap, but I have to confess that I don't know what it is: just being able to see at the moment of exposure, I suspect.
Fifth, SHOOT ANYWAY. Shoot at 1/8 if you have to. The shot may be blurry but it may not. If you don't play, you can't win. You can even 'bracket', shooting at 1/8, 1/15 and 1/30 wide open: decide later which gives the best compromise on sharpness and shadow detail.
Cheers,
Roger (and take a look at the Photo School on
www.rogerandfrances.com)