This is something we've put a bit of thought into!

Initially when we bought our house together we had one joint account, but found we were spending too much money on crap.

Now we have three bank accounts - ours, his and mine. Everything goes into the joint account and all the bills, mortgage and other agreed expenses come out of that account. Otherwise we don't touch the joint account.

We call our own bank accounts our 'unjustifiable' money. Every fortnight our 'allowances' get automatically transferred from our joint account into our unjustifiable accounts. Our allowances are now $300 per fortnight and that is our play money. I use mine to buy books, gym, lunch, movies, go out etc. Hence the 'unjustifiable' part of it. We don't have to justify this to each other.

We have found this to be a really good system. We talk about big purchases (heater for the bedroom being a recent one) and then buy that out of joint money.