Trent at The Simple Dollar has a post on this same topic. One thing he considers is placing what you would have paid extra to your mortgage into a savings account or ETF/Index fund, and letting it sit until the balance in that account is equal to your remaining mortgage. Then, pay it off!

This has several advantages: you get a better return on your money and so you accumulate faster than just paying off the principal; you still get the advantage of the full interest ductability; you have that money available to you in case of a dire emergency or to down pay on a new home if you sell early. The biggest con is that you are paying taxes on the interest/dividends you gain, and at some point in time that tax is actually higher than the benefit you get from deducting your interest.

I like this approach, though, and is likely the path I will take. Here's Trent's post .

-Mike