We live by Quicken! We use a "virtual envelope" system with Quicken and have a very detailed budget which we tweak every year. I can't imagine living any other way (although it took the first 6 months of our marriage for me to get used to DH's system!).
As PPs suggested, the first thing you have to do is write down every single penny you spend - especially the cash. And I mean every single penny, even that 60 cents in the soda machine! Once you see where the money is going you can determine how much you need for each category, and where you can save. It's amazing how much cash is just "lost" every month.
Off the top of my head, our major categories are: groceries, gas, entertainment, dining out, home maintenance, charity, utilities (electric, water, garbage, alarm system, etc.), mortgage, car insurance, phone, taxes, and we each have a monthly allowance. We budget in the extra food costs for Thanksgiving and Christmas by putting extra money in the grocery budget for November and December right off the top. We also put extra in the electric bill category for July and August because our A/C runs non-stop those months. Savings categories are also a must for us. We have categories for a new car, home improvement, vacation savings, Disney Passes

and of course retirement savings.
This system works great for us - we never argue about money because we discuss and agree upon how much goes into each category and we stick to it, we have no debt except for our mortgage, and we have savings. I've tried to explain this system to friends, but if both partners are not on the same page it will not work. It's not enough to set a budget of $400/month on groceries, for example, if one person is consistently spending $700/month on groceries.