A friend of mine claims to feel a whole lot happier now, since she decided that what she has no control over she should not get stressed about. That makes a lot of sense.
As regards letting OUT the stress that flowed in, how about a bit of visualisation. I visualise wrapping the stressful thoughts up in clingfilm & throwing it out of a window in my head. Though mind tricks like that have a habit of getting out of control, when you have to start thinking up elaborate reasons for why the parcel can't just fly back in the window again. Actually - maybe not such a good idea after all.
Deep breaths maybe?