I use a roll of toilet paper and leave markers where I've already searched. When the blood trail ends I usually start a tight spiral search. I've literally had to trip over a Deer to find it before in tall grass or weeds. Don't count on spotting a downed Deer from any distance. Grid search or spiral search keep it tight.

A good nose dog is always the best, the nose knows. They can smell invisible blood flecks.

If there is brush, look on the brush as well as the ground. I've found myself fixated on the ground before and missed smears on bush leaves.