Pred Diff issue?

[Randall34]

I ran my pred in anger for the first time today and it took about a minute for the diff (rear) to loosen, i tightened it and it happened again. I seem to be doing it up quite tight (too tight it seems) so that it feels very frictional but it still seems to undo. Is there a tip for keeping them done up or am i doing something wrong???
Can anyone assist me in my plight?
Neil