Ideally, you need to have regular shopping outings when the kids are with dad or a babysitter. You'll get so much more accomplished, and not feel so pressured to get everything done during those unavoidable times when you must shop with the kids.
Online Shopping. Moms today are fortunate to have this option, so take advantage of it, even if only to get ideas of where to look and what's out there. Use sites that have free shipping and returns, like Nordstrom and Zappos.
Budgeting. Keep a separate budget for kids clothes and your clothes. Make sure you actually spend your budget, and don't use it on the kids.