Hello,
Make sure that your IV arm is primed with saline first before starting your blood or if you are just using the blood to draw out of the arm for blood draws, thin it some with the saline. We always have some saline running with our blood so it doesn't clot.
If you are using your arm for blood draws and IV insertions you could have the blood on a pressure bag (low to med pressure) and that way it is always running to the collection bag. Then just put the pressure bag on the collection bag and restart again. That way it is a closed system and the blood doesn't stay still long enough to congeal.
I hope this is helpful.
Dani