Hi Chris,
What you need is so called 'DC-DC converter',
and you need a kind of conversion circuit that consists of an oscillator
and boost circuit. This is the only way you can get higher DC voltage from low DC input (here 0.9V as you said).
Try the following site where you can get some basic understanding
about its operation, and surf the internet for detailed schematics.
There's a bunch of them.
Just my 2 cents.
http://www.powerdesigners.com/InfoWeb/design_center/
articles/DC-DC/converter.shtm
Robert