TheApple

In the school that I work at it is really easy to find volunteers for everything. Many of our families have one parent who stays at home which definitely makes it easier!
As a school at the beginning of the year we have a parent carnival set up by the parent council. They make it a lot of fun for parents. On the first day of school, after drop off parents gather in the gym where there are tables set up. Each table has 2 or 3 volunteer opportunities for the year. They set up flags on each table that are brightly colored and say what volunteer opportunities are there. The tables are color coordinated by table cloth (kindergarten is one color, first a different, etc.) Parents get a guide as they come into the carnival describing each volunteer opportunity. They put sticky notes and pens on each table so that parents can make a note of what they have signed up for. It is a fun time for parents to meet, talk, and sign up to help in the classroom. The parent council has the whole affair catered with breakfast foods and the whole feel is kind of like a cocktail party. The parent council gathers all of the volunteer sign ups and sends them to the teacher. Teachers have a section in their weekly newsletters where they remind parents of what they have signed up for. As a school we also have a hallway bulletin board where two parents are honored each month as "volunteer of the month" they also get mentioned in our monthly school newsletter and a $10 gift card to the coffee shop Wild Blue (yes we have our own Starbucks).

Free food.  Just kididng.  I think that all parents want to feel appeciated.  If you let parents know how and why their volunteering is important, they will be more wiling to help out.