Practice “going without.” Talk about how nobody gets what they want all the time. We should learn how to just choose to go without something we want. Get them in on the discussion and pick something every day to do without.
Set up automatic payments so they are initiated on your end, so that you send out money to the bill/ company you’re paying instead of giving them access to take money out of your account.
"Watch the pennies and the dollars will take care of themselves." - Ben Franklin
This works with money and with other areas: Be faithful in the little things and the bigger things will take care of themselves. For example, if you don't get comfortable with telling little lies then you'll never tell big lies.
I once asked my mom if my grandmother (her mother in law) was rich. Her reply: “No, but she likes to think she is.” I think she meant my grandmother spent money like she had plenty of it when she really did not. Can relate. I hope you do better!
Misquoting someone here: “It’s not about getting everything you want, it’s about giving everything you have.” Stuff will never satisfy. Pouring yourself into something heart and soul will.
Have a yearly “review” right before school year. Raise allowance as appropriate. If they want more money, they’ll have to take on more responsibilities.