/dev/random is fed by an algorithm that does very much what you're
describing. It takes several variously-trusted entropy sources and
mixes them into an "entropy pool" using a polynomial function -- for
each new byte of entropy that comes in, it's xor'd into the pool, and
then the entire pool is stirred with the mixing function. 

When it's desired to get some randomness out of the pool, the entire pool is
hashed with SHA-1 to get the output, then the pool is mixed again
(and actually there's some more hashing, folding, and mutilating
going on to make sure that reversing the process is about as hard as
reversing SHA-1). At the same time, there's a bunch of accounting
going on -- each time some entropy is added to the pool, an estimate
of the number of bits of entropy it's worth is added to the account,
and each time some bytes are extracted from the pool, that number is
subtracted, and the random device will block (waiting on more
external entropy) if the account would go below zero. Of course, if
you use the "urandom" device, the blocking doesn't happen and the
pool simply keeps getting hashed and mixed to produce more bytes,
which turns it into a PRNG instead of an RNG.


Using a hash function is a good approach - just make sure you underestimate the amount of entropy each source contributes, so that if you are right about one or more of them being less than totally random, you haven't weakened your key unduly.


http://security.stackexchange.com/questions/2604/evaluating-the-entropy-gathering-in-a-prng


http://stackoverflow.com/questions/3436376/what-is-the-most-secure-seed-for-random-number-generation/3532136#3532136

https://www.schneier.com/blog/archives/2006/06/random_number_g.html

http://www.howtodothings.com/computers/a1337-cryptographic-random-numbers-.html

https://www.merrymeet.com/jon/usingrandom.html

http://www.astro.umd.edu/~chris/publications/html_papers/deadsources_revised/node9.html

http://crypto-how-to.com/entropy/