In the past I used to keep it in, but its not the most healthiest way to deal with it, in tends to turn in on you.
I let it out now by doing the following; going for a motorcycle ride, a run, kicking and punching a punching bag (and occasionally a human, in the ring of course) throwing empty beer bottles at a brick wall as hard as I can (I clean up the mess afterwards), going fishing and I write about my feelings and what triggered my anger so I can learn from it when I calm down.
I let it out now by doing the following; going for a motorcycle ride, a run, kicking and punching a punching bag (and occasionally a human, in the ring of course) throwing empty beer bottles at a brick wall as hard as I can (I clean up the mess afterwards), going fishing and I write about my feelings and what triggered my anger so I can learn from it when I calm down.