You should try with BUCK-BOOST DC/DC converter. There are available with efficiency above 90%
Check out TI and Linear websites; there are "calculators" that would help you:

 - http://www.ti.com/lsds/ti/analog/powermanagement/power_portal.page
 - http://parametric.linear.com/buck-boost_regulator

Options:

 - [TPS63031](http://www.ti.com/lit/ds/symlink/tps63031.pdf)
 - [TPS63001](http://www.ti.com/lit/ds/symlink/tps63001.pdf)