Here's what I expect parents to do and what I do myself.
Costumes
I find the costumes, do the measurements, the ordering, and find the 'little bits' that might need to be added to the costumes. I ask the parents to make sure the costumes truly fit (by taking in the shoulder straps), and to add the 'little bits'. For an explanation why, it's cost. I can get a basic tutu without adornments for $15 and for the extra $5 for bits to be added. If the parents does the rest, that's $20 for them for the costume. To buy a fully loaded tutu for a four year old, it's going to be at least $50, and the parent still have to adjust the tutu in the shoulders, length, and/or waist.
For costumes that belong to the school, it will depend on the type of costume. Simple leotard and skirt? Parents, please adjust the straps of the leotard so that they don't fall down. Complicated costumes I have a seamstress to do the adjustments, but that fact is reflected in a more substantial rental charge, since I have to pay the seamstress.
Sometimes parents are asked to make simple headpieces.
Volunteering at the show: I ask that every family provides one family member to do one thing. In exchange they get two free tickets. No volunteer? no free tickets.
Backstage moms are the most important thing to have, and the most difficult to get. I had a mom ask me one time why they had to do it. My response was that there had to be someone backstage to make sure the little ones are taken care of during their downtime, we could find babysitters, but 1) thats an additional cost that would need to be passed on to the parents, and 2) do you want strangers watching your children backstage?
We also need people to work the box office and refreshments. I gave up on asking dads to help with rolling out our dance floor. Instead I pay the high school theater techs to do it, with that additional cost being reflected in $1 raise in ticket prices that year.
I do usually use alumni students to help the older students with quick changes and gopher activities.
Thoughts
I have to say that anytime I ask someone to help out, I feel guilty, guilty, guilty. Like I am doing something wrong if I have to ask parents to help out. I keep having to remind myself of two things.
1- It's a long standing tradition, not only in the arts, but also in sports and academic activities, that parents help out.
2- Parents who are involved have children who remain involved. I usually know that a student won't be coming back the next year when a parent refuses (without good reason) to helping out.