I would suggest that in the early season, when it is warm, look for ants. I've almost always seen ants on my blood trails when tracking during early season. It doesn't seem to take long for them to find the trail, either.

I also agree with alot of what Matt/PA wrote. Use your other senses. Alot of times, I've heard the deer "crash" even if I couldn't actually see them fall.

Slice.