add as much variety as you can, rocks in some sections and smooth jumpy sections ,and mix it up with corners ,have some berms but also some flat corners
walk the area thoroughly before you build taking note of natural features that you can factor in,this i believe is the secret of ''flow'' as the hill is offering its opinion too.As said before drainage,what good is a trail thet takes a month to repair after each good deluge?(put a drain in every switchback/berm and every 50 or so feet on steep sections,water bars are cool cos youcan jump them too).for features i'd have rock gardens,doubles(but not with too much kick as that will slow you down) a cattle grid, a couple of drops and a chute with a bit of distance between so you can hit them fast
a varity of sweeping corners just on the threshold of traction. some fast other slower, some steady state so the same tightness right round, and others dynamic that get tighter on you or widen pefectly if you hit the beginning with speed.