The first thing I would say, although this is obvious with any sinew backed bow, is to make sure the wood is very dry. Hackberry seems to me to be a wood that needs to be very dry to take little set. After the bow is ready to be floor tillered, you could use a hotbox, or you could also throw it in a hot car in the sun all day long. The second thing, although once again obvious for a sinew backed bow, is to make sure you reflex the bow a good deal before sinewing to counter set. This will end up pulling it into reflex later, even if the bow gains some set initially.
