[pushkins]

Venting via the roof is the best option as moist air being warm, rises, many people however are a little wary of cutting a hole into an otherwise perfect roof.
Venting via the soffit is an acceptable way as well, you can buy soffit vent kits at most big box stores and they are very easy to install. I don't think you need worry too much about the soffit drawing the moisture back in as the vent flow will push the air out far enough that natural air currents will disburse the moisture, and what little if any does make it's way back into the attic space, will not be a drop in the ocean compared to the standard attic moisture levels.