What is the most important skill a volunteer attending a blood donation drive must possess?

Concern for the Blood Donor.

Ensuring Standard Practices are adhered to by Blood Bank

Should reach the venue in time. Being early for the venue almost always helps in sorting out logistics and other minor issues.

Should have good negotiating skills.

A good eye for detail so that issues that can occur, if any should be known beforehand.

Should be bold enough to speak to a crowd if required.

Ability to talk freely to anybody during the drive. He/she should not hesitate to talk to people in canteen or donors in the camp.

He/she should carry a pen and paper and use these very frequently during the camp. I mean that every thing that can be noted down as an improvement or as a feedback should be noted down ASAP in that paper. 

1. He should be motivating enough for the donors.

2. He must be good at troubleshooting random problems.

3. He should always look out for potential threats which may spoil the camp.

4. He must be hard working.

5. He must always smile while speaking to company contact / blood bank / donors.

A very powerful skill of a volunteer in a blood donation drive is a persistent, warm and genuine smile. The smile involves donors, makes communication easy, reduces tension levels and creates a nice environment for things to proceed smoothly. Each donor comes fresh.. let the volunteer be there with the refreshing smile :-) !

Persistence is another non-negotiable skill. Whatever happens, persistently driving the correct methodology is extremely important. Know that there are people to be kept safe. 

Revisiting various functional aspects of the drive (that you have taken responsibility for) repeatedly in a small cyclic manner ensures that nothing get's left out.

Having an eye for cleanliness and orderliness definitely helps. If the layout of the drive is systematic then people go from one table to the other without getting confused. The way chairs are arranged could make the difference between too many people standing or sitting. Papers scattered on the tables could make it difficult for you to find the stuff when you need it. Paper cups, plastic wrappers and cotton swabs - if littered around  the place make the place uninviting.

If things are in place very well through the drive then one can figure out potential problems very easily. This includes early identification of post donation reactions, monitoring involved people and ensuring that no one is missing any critical step in the process.

I had put some 5 comments yesterday ,, none is visible here now :(

I would like to add more:

• Volunteer should be highly energetic and motivated throughout the drive. He/she should enjoy volunteering in Drive.
• Drive in never COMPLETE until feedbacks are properly discussed and communicated to appropriate stakeholder. So, volunteer should be willing to put effort after drive.
• Should have well understanding to "medical science of blood donation". It helps a lot. Many issues may not arise at all if there is proper understanding of Medical Science. 
• Should have good understanding of  difference b/w Replacement and Voluntary Blood Donation.