another tip would be don't overwhelm the parents you do get to sign up. If they volunteer for one event, express there are other events but don't expect them to do all of them. They will burn out quickly.
I am the first volunteer coordinator our schools has had. I volunteered to do the job because in the past I volunteered for things and was never contacted. I think alot of parents feel like they are on the outside of a tight group. The more open and friendly you can make the group sound the more willing parents are to sign up to help. The PTSO at our school agreed to purchase PTO Manager software this year. It has been wonderful!! I customized the parent interest form for our school and sent home with kids. When an event comes up I email everyone and schedule them for the events. I have had so many new parents sign up and so many PTSO people comment on how wonderful the new system is. We also did "Welcome Bags" for the kinder and preschool parents which were very well received. Email is wonderful, no pressure!
One thing you need to remember is smile and be at ease!!!!
Meet people, network, encourage the point they dont need to volunteer for the whole event. Go outside the box. There are lots of elderly people that just sit around their apts. all day long. Volunteers don't need to just be parents. Try aunts, uncles, older siblings. At our school district, the high school has a program in the government classes that the students need so much volunteer hours to graduate. (they call ours P.I.G. particpants in government)
Try something new! Offer a volunteer dinner or I read an entry on here, where one school does pride points for the students.
What types of activities are you lacking volunteers? Can you describe your PTO and your school -- is apathy typical? If not, what has changed? If so, what has been tried in the past? How are you getting the word out that volunteers are needed? What are you hearing in response?