Quelques suggestions (qui ne corresponent pas forcément au niveau de langue) :
Don't get your knickers in a twist.
There's no reason to have a conniption.
There's no reason to get worked up.
Take a chill pill.
Don't have a cow man.
It's not worth a hernia.
No reason to flip out.
Nice list Aupick, that'll cheer us up ! After 'broyer du noir' tonight, we've now moved into full crisis mode. I like the commonly used (BE ?) 'to throw a wobbly': to have a fit of temper, irrational behaviour, or great agitation.