The "how"--
My account handles my income (direct deposit) and all expenses that have a bill attached (they are all set up for auto drafting). I also keep a small petty cash fund that I replenish monthly to an agreed upon small amount. Monthly I transfer an amount to her account to balance her side of the expenses, and the rest goes to an investment account.
Her account handles her income (direct deposit) and all expenses that do not have a bill attached, e.g. groceries, clothing, doctor visits, gifts, vacations, etc. She also has a small petty cash fund that is replenished monthly to an agreed upon amount. Her account is for expenses only, no savings contributions.
The "why"--
I can live off nearly nothing for a year and when I spend money outside a budget, it is for a larger-priced item that I have researched forever, waited for more than a year to buy, and usually found on Craigslist for 40-60% of the new price. Eleven of twelve months each year would be almost identical for me, and the other month would also be in line except for one larger purchase. On the other hand, the DW will trickle money like a dripping faucet, in $5s, $10s and $20s all over town every month. Previously, tracking and planning was a nightmare. To be fair, she'll give $50 away on a gift more quickly than she'll spend $50 on herself, but the end result is still $50 that is gone even if we've already given to charity or church for the month. Our system took several years to iron out, but now we're in sync. She has agreed money to spend as she wishes within a limit (and she looks harder for bargains since we've agreed she can have the extra there as well--i.e. family commission). And now I have consistent monthly numbers I can use to plan for the future.
Bottom line--Any system that keeps financial arguments out of the marriage while intelligently providing for the overall financial well-being of the family strengthens the marriage.