Youtube - any tips for putting videos up?

[bizzylizzy]

We've made a part live action, part animated video to put up onto youtube. All the editing etc was done on a mac. Having rendered it, running it in the same format that youtube require we're suffering from quite a bit of pixelation in places and odd changes of colour. Same goes for producing a disk. The Mac one's OK, but we've having probs producing the one for PC. Any tips, hints etc would be gratefully received.