Rule #2 of the You Need A Budget Method is about saving for your true expenses: things that you know are coming up, but still act surprised when they do. (ie: Oh crap, I have to pay registration this month?!?!)
Saving ahead of time allows you to have the money when it's time and you won't be set back.
Most people will have similar true expenses - car registration, birthdays, holidays, taxes - but many of them are personal: renewals, children's activity fees, personalized home/auto services, etc.
Start by writing out everything you pay for each month that is NOT a fixed expense like mortgage, daycare, or insurance. I went back through my calendar and bank statements to get very specific.
Guesstimate a dollar amount. Add up the month to know how much extra you will need each month and start funding a "True Expenses" category. Or you can get very specific and have savings envelopes for each expense.
No matter how you decide to save up for these future expenses, it's crucial to first know what they are and just start sticking away money!