Ugh - I have no idea why my first post disappeared and then reappeared, and why I was able to post 2 rather than just edit the first! So sorry to all!
You need a good advertising program so you are visible and have a fun name. We used "Big Bucks" but you could do anything to promote the school needs that will be met - Playground Fund or Enrichment Fund or Field Trip Fund or whatever will make people realize it's not going into the general fund that pays for floor cleaner for the janitor. (Floor cleaner is important but it's not glamorous!).
Then you need a way to remind people - that means being incredibly visible. You need clever email flyers with the benefits listed in bullet points. See if you have a good conceptual advertising person among your parents. That's NOT just a graphic artist who does cute designs - it's an IDEA person who keeps things compelling yet simple. Then have an advertising art director do the layouts. If you come up with a cute name, a graphic artist might design a logo for you. Maybe a local ad agency will do something for free for you - then give them credit in your newsletters and parent emails, take a photo with them and you for the local newspaper, etc. You also want to expand your buying public beyond the schools - you want the town citizens to support the program. That means better publicity and having parent "ambassadors" spread the word and get their friends/relatives to buy in.
Then be visible at every possible event - parent open house, parent conference days, kindergarten screening days, field day, book fairs, school and town fall festivals, assemblies/performances, and so on. You need a table/booth and good volunteers. If you can do something at the parent pick-up or drop-off line without messing up the flow, do that.
What scrip are you using? Supermarkets are ideal because everyone shops, not just for groceries for but prescriptions, postage stamps, carpet shampoo machines, and other gift cards. If you use supermarkets, get them to let you set up a display/sales table there as well. People who buy scrip usually spend more so it's good for the markets, plus they want the publicity of supporting local schools. Remember that your supporters don't have to be parents - they can be other relatives, friends, and townspeople who want to support the schools.
We did a mail system as well, either as a "first time" convenience or for regular, reliable participants. Mail the gift card(s) in whatever amount the customer designates, keeping a careful record of the serial number(s) of the card(s) and the date it was mailed. See if you can get a local paper company to donate leftover envelopes in a bright, fluorescent color so the recipients see them. You could enclose a return envelope, either from the donated stack or a stamped self-addressed envelope provided by the customers. You don't want to pay postage in both directions if you don't have to. You need a really dedicated, small, vigilant committee to keep track of the cards out and checks in, and you can't send new cards to someone who owes money!
See if you can sweeten the pot a little by getting local merchants to donate gift certificates (hair and nail salons are big ones, but so are restaurants and independently owned gift and home-improvement stores. Big chain stores have a certain amount of community dollars to give away too - ask for the manager early in the month since many are on a monthly budget for giveaways.). Then design a contest for people to earn these gifts - the most new people signed up by a volunteer, a drawing among all new people (new people qualify for one of 5 prizes), first 10 people to enter the monthly mailing program, most consistent customers, etc., whatever you want. Think about whether you want to reward the biggest amounts purchased - that kind of hurts people on extremely fixed incomes. But a referral system for the most new customers might be a great thing that anyone can participate in.
People need to understand that this program costs them nothing. It's what they'd be buying anyway. That's why basic purchases entice people more than gift cards to places like Amazon or Land's End that they aren't using right now. Be sure you thank new participants as well as volunteers in every issue of the school newsletter. You may be able to thank volunteers on the school website too.
Good luck!