We took the expenses over the course of the year and divided by twelve. then we added that to any big expenditures coming up, and put it in a separate savings account, where we could not access it easily. We tend to forecast our expenditures 6 months into the future, if possible (some things just break down without warning!).
I like to put money aside right when we get paid. It wasn't much at first...maybe a few bucks. I also build it into our budgeting system. DH needs $80 for gas for the car and an average of $80 for repairs a month. So he gets $160 in the transportation budget to cover gas and repairs. If he were a spender (which he isn't), I'd be putting the $80 for repairs in a separate account that was hard to access.
We also buy a lot of our big ticket items used or secondhand. I'm looking at a nice 15 cu. ft. freezer right now that I bought for $75, and a can of Tremclad spray paint ($5.92).
So I guess I attack this situation from two angles: I put aside extra money a month off the top, and I buy used whenever possible.
Jean