I try to mix it up. I used to just read "what I felt like". But, I almost always wanted to read SF, sometimes fantasy. Now, I make sure to alternate between SF/fantasy and "other stuff". I also try to balance out "light" and "heavy".
"Other stuff" includes history, philosophy, religion(mostly eastern), classics of literature, and generally things that have a strange point of view
History: Just finished the second in a trilogy about the Third Reich by Richard Evans.
Philosophy/(religion?): Currently reading a novel by Hermann Hesse. He usually writes a sort of Western view of Eastern philosophy/religion. This novel(Narcisuss and Goldmund) discusses a philosophy of duality, though maybe like Steppenwolf the main character may change his viewpoint.
Classics of literature: most recent was Dostoyevsky's The Brothers Karamazov.
"strange point of view stuff": while my taste in SF/fantasy tends toward this, the best example of it I can think of is Freakonomics. I like to get my mind blown.
Science Fiction/Fantasy: I either like to laugh or get my mind blown. Books don't often do both. Currently reading Old Man's War by Scalzi. The main characters are all the same, otherwise ok. Last great one I read was Olympos by Simmons.
My "light" tastes might be something like Pratchett. Some old school SF fits this, but a lot of old school SF writers can't do characterization. Some are too linear in plot development, too. My "light" is almost always SF/fantasy.
My "heavy" might be something like Dostoyevsky/Frank Herbert.