how can i stop my treadmill belt slipping?

i have a dynamix treadmill - one which doesn't have a motor....
the belt on my treadmill slips to one side after about 1 minute of running/2 minutes of walking every time i use it.
if i tighten it it slips to the other side and so on, at the moment i just start, have to get off reset the belt, start again get off etc.

any help?? i have no idea about this kind of stuff so any help is muchly appreciated!!!!! thankyou!