As the title implys I have a question about up/down strumming.. are there any excercise that I could avail of to improve this?

Or is it all just down to sheer force of will and practice?
Just up and down? If you're having trouble with timing or something you could take it slow and use a metronome. What exactly are you having trouble with?

EDIT: I think this would be better off in guitar and bass basics.
Well I'm having a bit of trouble with my hand positioning...

For example when I'm trying to play the main riff to MoP I'll play --01-- on the 'E' string

0 - down
1 - up

Then when I'm going to hit --2-- on the 'A' string I'm just pretty much out of position and end up hitting the --0-- on the 'E' string again.. it then just puts me off completely..

I'm not too bad with the timing but will take your advice and start slow with the metronome.

Any other would advice would be much appreciated.
Practice practice practice - its the only answer especially if your teaching yourself. Develop your own technique as long as it sounds right.