Add 5 feet to the stack to give it better draft, and top it with either a venturi-type or whirlybird-type wind turbine so that even the slightest wind gives it a stronger draw instead of a backdraft. These people have a decent selection of vent turbines, etc., but they have plenty of other competition. If you're going for a whirlybird sure to pick an all-metal version (some have plastic bearings) if it's to be used on a chimney.
If it's next to a gas or propane flue in the same chimney you need to go stainless or it'll corrode quickly, otherwise copper or galvanized both work for wood-fires only.
I prefer lower-performance but lower maintenance venturi types myself. Too much suck and you de-pressurize the house, increasing outside air infiltration. Something that looks like these is usually enough to make a difference, along with a few feet of extra stack: