This is the easy part, the hard part is actually actioning this across you and your spouses 401k, Traditional IRA, and Roth IRA (god forbid you have 401ks from previous employers as well). If you are lucky enough to have the same funds across them all, it helps a little bit.
As an example, I target a portfolio with 85% stock, 15% bongs. Of the stock, I'd like at least 35% international. So, when making our IRA contribution for 2013, I need to get asset allocations of all funds and their balances, from all accounts. Figure out my total allocation and then figure out where my additional funds should go to maintain or rebalance to my target allocation.
There are some tools for this [1], but none are great. I've considered building a tool to do it myself, even started on one, but getting the asset allocation data costs and I'm not sure that there is a real business opportunity for an app. This leads to increasingly convoluted spreadsheets for managing the process.
Target funds can also help, but you typically get better expense ratios for doing it yourself. Though, I've recently considered just giving up an moving entirely to target funds.
[1] http://portfolio.morningstar.com/Rtport/Free/InstantXRayDEnt...
