I like this post, Omni.

Hopefully people will post some neat tricks.
I just breathe in slow and deep filling my belly first and then my chest. Then I exhale slowly letting out the air in my chest first and then my belly. It's a nice way to clear the mind before bedtime.
While scuba diving (a little tip an instructor told us) was to count the length of your breaths. Shoot for the count of four and work your way up. It saves air and extends your dive time. It also helps when you just need a little more oxygen in the blood stream.