How should one prepare beef heart?
If it's from an animal that is "young at heart" (veal/calf), it may be tender enough to cut into thin strips and pan-fry or grill quickly as "steaks."  Don't overcook, or they will be really tough, and you'll be heartbroken.

But to get to the heart of the matter, if it's really an adult beef heart, it's too tough and chewy to cook quickly.  Braising is my preferred method and the one closest to my heart.  This is one of those things can cook all day and will eventually become incredibly tender -- tasting like a slightly unusual and hearty pot roast.

You can also stuff them and then either braise or roast them -- again, "low and slow" heat is the most heart-friendly.  If you roast it, also be sure there is enough moisture in the stuffing (i.e., leave a "soft spot" in your heart), or else it can dry out during a long bake.  (You can also "roast" it in a dutch oven or something.)  When you open up your heart after cooking, a dry stuffing will just leave you with a heavy heart.

A final option is to use it as stew meat.  It will take longer to cook than most stew meats, so you might want to simmer for a while first before adding vegetables.  Then cook until the flavors come together and your heart's content.

Once, when I had leftovers after making a stuffed heart, I was just going eat it, but then I had a change of heart.  I chopped up the tender meat and added it to chili.  It probably was about 1/3 of the overall meat in the chili, along with some diced chuck chunks and some ground beef.  It came out incredibly good: there was a deep earthy flavor that really worked well.  I served it to a number of friends without explaining the dish, and they didn't notice anything unusual; I didn't have the heart to tell them.  In fact, a number of people said it was some of the best chili they had tasted, which was really heartwarming to hear.

Take heart!  There are lots of ways to cook it.  Any slow-cooking method or recipe you might use for a tough cut of beef will probably work -- just follow your heart.

(I am actually serious about all of these methods -- but, okay, truly sorry about the puns.  From the bottom of my....)