I don't usually eat Beef heart, but we eat Lambs hearts reguarly (my kids love them). I take out all the pipes and sinews with a pair of kitchen scissors, and create a large cavity. I stuff it with a homemade breadcrumb-based stuffing (finely chopped onion, breadcrumbs, suet, egg to bind, herbs and seasoning) and roast in the oven for about 30-40 minutes. I then slice them and serve as a roast. I also put them into my cookpot. I prepare them in the same way, but don't stuff them. Add a red wine stock, and we also add whole peeled pickling onions (small shallots) and mushrooms. Cook in the pot until tender (I give them about 3 hours). Slice them and serve.
I find placing a whole heart on a plate puts some people off. Slicing them through is a nicer way to serve them. Try Lambs hearts if you can get them - they have a very delicate flavour. Avoid Pigs hearts unless you really like them, as they have a very strong - some would call overpowering - flavour.