Base Conversions

[RandomPscho]

Hey all,
I have to make a program that converts from any base number system to another. I know how to do anything to base ten, but how do you go from anything to anything?

Going to base ten is simple:

digit3 * base^2 + digit2 *base^1 +digit1*base^0 etc;
but how do you set it up to go from other bases? I know its easy, but I can't think of the setup -.-

Thanks.