1. Show them your total debt.
Depending on your debt load, this ought to illicit enough panic to convince your family to never stray from the budget again.
2. Be prepared.
For us our biggest struggle is eating out. Its just too dang convenient when we are both working and have a little guy at home to take care of. If we prep our meals (so all we have to do is heat it up when we get home) or even just write down what we are planning to eat for dinner each night, we are much more likely to stay on track. When spouse calls hangry on his/her way home from work, you can assure he/she that dinner is ready. Wallets and waist line will thank you.
3. Have a realistic budget.
It would be really nice if we could NEVER buy gas, or clothes, or eat things besides Ramen noodles, but the truth is, its just not going to happen for us. Life happens. We have to drive to work, no one will let us walk around naked, and I think we'd die of malnutrition if all we ate was Ramen.
In my experience, setting and sticking to a budget takes a bit of work and we are constantly having to re-evaluate and reassess our expenses. Some months, we need more money for household/cleaning items,etc. Some months we need more baby things. Be flexible, but try to keep your overall monthly expenses the same. Which leads me to #4...
4. Have a budget that works for you.
What has worked best for us is to have a total monthly budget to stay within, instead of allocating specific amounts to specific areas ($200 for gas, $300 for food, or whatever) Instead we just say we have $2500 a month to spend on everything- rent, gas. food etc etc. Some people might do better with a more strict budget that allocates money to distinct areas. Do what works!
5. If all else fails, have them go to a collections agency hearing at your local courthouse.
Easy for me to say since I see them on the regular. But it's truly horrifying every time. :)