I tend to use should when it's a suggestion I don't have a strong opinion on, i.e. it could be done in many other ways than the one I'm suggesting and it can still happen.
You should stop by that gas station to go to the bathroom.
On the other hand, I usually use must for things that have to happen in a certain way; any other way would make that thing not to happen. The problem is that some people find this usage harsh, as they receive it like if I'm giving them an order.
You must cross the street to get to that store.
To me, it's just a depiction of reality, not an order. But I'm neither a native speaker nor a regular person. :-)
So, what's the proper use for must and should? Does this use differ among the English dialects?