Something that works well in the field are desiccant pouches.
You can buy them online for less than $5 each for some good sized pouches. I use Desi-Pak.
In the field, when you're sleeping or anytime you can be without your boots on your feet, throw a pouch down in the boot so it lays flat on the insole, draw up the laces tight, fold over the top edge and tie them so they're almost airtight.
The pouches absorb water from the air so you want the boot closed to keep the pouch from absorbing from anything but the wet boot.
Drying boots too quickly can cause them to shrink, crack, become brittle, or even melt the sole depending on what type of rubber it's made of.
Raith told me about this a few years ago, but I didn't try it until we were in the field once at Ft. Hood. We use DesiPaks for the TADS PNVS bucket, so I snagged a few before we left for this purpose. My boots were perfect when I woke up every morning!
TIP: Be sure to store them in a ziploc bag when not in use. If they soak up alot of moisture, put them on a cookie sheet and bake them at 150 until they feel light again. A microwave can be used in a pinch, but doesn't work as well.
This will keeps your boots dry on the inside, where it matters, is reusable, and is portable enough to throw in a ruck for field use.
edit: here you go:
http://www.cheaperthandirt.com/LOW602-1828-1395.html
