Betterment makes it super easy and *constantly* re-balances with EVERY dollar you invest, whether through deposits of fractional shares or reinvestment of dividends in under-performing assets. The constantly well-allocated portfolio, complex tax-loss harvesting (for my non-retirement accounts), and the lack of cash drag are what make it my platform of choice, outside of the TSP.
Give it a serious look. The fees are pretty low.
Note sure if this is allowed, and you should investigate it seriously either way, but here's my referral link: betterment.com/invite/evanoxhorn
In the alternate, MMM has a post on the service, I believe, and you can click through his referral.
Simply put, it's the best 90% solution around.