here are some ideas:
If all you need is to increase voltage you can build a voltage multiplier with some diodes and capacitors. I dont remember the exact circuit, but you can find it in any book that describes diodes.
Consider building a regulated power supply, eliminates problem of dealing with batteries.
Check out this link, it shows you a circuit where you can control output voltage.
**broken link removed**
Do some research in the internet.
hope it helps