well the rule that I try to live by and its not necessarily a frugal rule is "Life is so much easier when you follow the rules".
thinking about it I guess it can be a frugal rule. Just as an easy example having your car registered, insured, license renewed, emissions tested if required etc. If you don't do things when required its only going to cost you more time and money in the long run. Like when you get pulled over for rolling through a stop sign and you let your registration expire. Well now you not only have to pay the registration, now you have a ticket and if really unlucky a court appearance with all sorts of fines attached to it.
One of the most basic things I have learned on FV is less is more.
Phantom electricity, know when your off peak hours are, use timers when possible, plan ahead and pay cash, get organized, read the local paper for free town activities, use your local library.
I cook in large batches which originally I did this to save time, it has the added benefit of saving money.
I've always spent within my means but I really got serious with cutting expenses and saving when I got divorced two years ago and it was the first time in my life that I was 100% dependent on my paycheck. It was a pretty scary realization let me tell you!
I have scrimped and saved like a crazy person where sometimes I really feel poor. But now, 2 1/2 years post divorce, I have money in investments, I have a nice 529 plan started for my son, I'm saving for retirement, I'm a couple a months away from having a fully funded 8 month EF. I was able to buy a house on my own with a 20% deposit and a great interest rate.
I'm starting to see my hard work pay off. I'm really on a natural high these days with all of the good fortune that has come my way because of the hard work, determination and focus I've had.