Two things works for me:
Nature.
Wear suitable clothing and walk through the bush, forests, along beaches.
Take your time and let your mind wander.
Sit down once in a while, take a deep breath and look around.
Notice the sounds and smells around you.
The birds overhead, the colours of the leaves in trees as they get blown about in the breeze.
The different smells. Does it smell like wood or of rotting tree litter?
Can you see any other animals around?
Feel the texture of the rock you're sitting on, it was made millions of years ago and will still be here long after we're all gone.
Grab a handful of soil. Is it moist? Can you see any insects in it?
These are the stimuli you have evolved to be sensing. Notice there's no TV, no mobile phone to distract you.
Take some more deep breaths and with each breath consciously relax different parts of your body.
Music.
This is a good one you can do at home, or anyplace else really.
Find some orchestral music and lie back in a dimly lit room.
Let your mind wander to the sound. The light quieter parts can make you feel weightless and float.
If your mind is still too loud turn the music up louder and try again. Reduce the volume after 20 minutes or so once your brain waves have synchronised and slowed down.
Alternatively - presuming you have internet access, you are logged into Wrong Planet after all. Go on a YouTube bender. Find a song you liked high school and haven't listened to for years. Play it and think about the things in your life back then. Does the song remind you of anyone? What were you doing back then when you heard it? YouTube throws up a list on the side of the page of similar music from around the same time, pick one of these out and play it next. You'll find it easy to find your groove in some old favourites and your mood improves.