With your spouse or partner, make a pact that you will not spend more than $XX without consulting each other. This will prevent impulse spending, give yourself time to rethink a large purchase, and allow another point of view before dishing out a large sum. Your partner may also remind you that a large bill (car or life insurance, for example) is due that you had forgotten about. Together you can.... (click title for more info)
negotiate a reasonable amount based on your household income. For some couples, that might be $50, for others $500 or $5000. If you find it hard to compromise, each individual can write down the high & low amounts they find reasonable. Then compromise on a middle figure.