Ironically, I'm best able to clear my mind by first filling it up with things completely unrelated to what's already in there, particularly if it's someplace away from where I normally spend time.
For example, recently I spent gorgeous autumn day at the state zoological park. Just watching the animals and exhibits for a full day, and immersing myself in the entire theme of wildlife and such just did the trick for clearing my mind of everything else. (It's a topic I personally enjoy, so there you go.) You'd be amazed at how therapeutic watching a seal swim in circles can be!
Similarly, spending a couple hours at an art museum can be refreshing as well.