...a good 'text book' solution from sinha
To improve on that, set the flash 1 to 2 f-stops *less* than the camera. This will provide a fill flash affect and more natural result (as suggested by dlc)
Should your flash allow it, try using "A" instead of "M" as sinha suggests (an advanced flash will be required, not all models give full control over the aperture setting)
If you don't have an ND filter, simply close down the camera's aperture until the shutter's sync speed (and a 'correct' exposure) is arrived at - remember to adjust your flash accordingly
Personally, i prefer the option suggested by metroshane: reflecting light back onto the subject gives me a pleasant, satisfying result
e_
EDIT: (a clarification) ... my reference to setting the flash 1 to 2 f-stops less than the camera is an area of confusion for many people
To illustrate:
Say, for example, that metering the scene calls for an aperture of f-11 on the camera
To effect 1 to 2 f-stops *less* on the flash means the flash setting will be either f-8 (less 1 stop) or f-5.6 (less 2 stops)
Yes, it's confusing - but that's photography!