I found this great idea in Dutch magazine home & garden. Fill heart shaped forms (e.g. baking forms) with water. You can add some twigs or berries for a bit of colour. Keep the forms in a freezer and wait till a cold, freezing winter day. Then remove the hearts from the forms, and make a hole in the middle with a heated nail or knitting needle. Pull a piece of string or ribbon trough the hole and voila, the hearts are ready to be hung on some branches in your garden.