You can learn biofeedback methods to do so. Bored on nightshift I have wired myself up to monitors to do so – can vary my pulse =/- 20% and drop my bp ten points or so. Its just learning to relax properly I guess.
Why do you want to do this? Actually alcohol does IIRC as do some recreational drugs.
How about meditation, works for me. All you have to do is sit somewhere comfy and concetrate on your breathing – in through your nose and out through your mouth. You dont need to do any silly mantras or that sort of stuf, just find something to focus your mind on something other than the everyday nonsense that stresses you out.