I personally hate stuff thats just clipped to the outside of the pack (or threaded on a strap) so it can flop about ,its hard enough walking with a pack on without having things swaying around (and the noise of a tin cup tapping on something really gets up my nose.....worse if its someone elses
)
As most of the guys have said "Need last, In first", but if there are two things you need at about the same time put the heavier one nearer your back or on top
I used to have a 4 pint water flask that got strapped-down on the top of my pack for general walking that got moved to the front when the ground was rough (I also learnt to keep it topped up so it didn't slosh too much)