I hate to put exhaust ducts through roofs, because there will always be some condensation that could drip back inside. I would much rather go through a side wall, or down and out the side of the basement. But if you must go up and through the roof...
Don't allow your ducting to have any loops in it -- they will collect condensation and mould in the low spots.
Use insulated ducting, or wrap insulation around ridged ducting.
Don't lead the exhaust duct out through the soffit area -- hot air rises and the warm moist air will simply rise back up inside the attic. And don't just jam the exhaust duct into a roof vent -- the vent hood will definitely fill up with condensation, which will drip back into the attic, or back down the exhaust duct. Instead, use special roof exhaust hoods that have a round collar the same size as your exhaust ducting, allowing you to fasten it to the hood solidly and air tight. Then choose exhaust hoods which have internal drain paths that ensure condensation is channeled out onto the roof, not back into the attic.