First, you need to cut a lot of blood vessels. This is shot placement and blade sharpness. (tho' the inner workings of a deer are not evenly vascularized, not even from one deer to the next. A halfinch one way or another can mean all the difference in catching a big artery or vein. This is why some lung/liver hits bleed like crazy and others don't.) I personally aim for the heart.

Second, you need holes for the blood to come out. Again shot placement. Low exits that aren't through gut or heavy muscle are the best. I personally aim for a heart area exit.

Did I mention shot placement and blade sharpness? (and aiming for the heart exit?) hee hee

Happily, most of my deer drop within sight. It makes me lazy on blood trailing, but I like it better.

Picture below is of the one deer that I didn't see drop last season. (a dorky 1.5 year old buck) He only went about 75 yards but the vegetation was pretty thick then. Anyhow, the blood trail was great.

(hole was made by a muzzy 125 grain 3-blade broadhead.... before anybody thinks I'm on the Rage bandwagon.)