Hugging and having sex are both known ways of increasing oxytocin naturally. This isn't just good if you want to bond with someone - raised oxytocin levels are associated with a decrease in anxiety levels and stressful hormones such as cortisol, and also lower blood pressure and a decrease in likelihood of heart disease, strokes, and other stress related diseases.
If you want to raise your oxytocin levels naturally, then take your social relationships seriously and become a more touchy-feely type of person. Don't be shy to give your friends and acquaintances a big hug every time you see them, or at the very least, a warm handshake. And remember that you can't have too much fun in the bedroom - good sex with a loving partner every day will help keep the doctor away!