You will need:
Something to smoke
Dexterous Lips

1. Read, learn and master How to blow smoke rings
2. You'll probably won't want to embarrass yourself. So find somewhere private where the air around you doesn't move.
3. Now for the technique: Hold the middle of your bottom lip slightly upwards. Some people find this totally impossible - I afraid they'll have to use one of their hands to press their bottom lip in that position. You can also hold the middle of your top lip downwards for the same effect, except your heart shaped smoke rings will be upside-down.
4. Blow smoke rings as normal - you should get something resembling a heart shape.
5. Practise until perfect (or lung cancer starts to take effect). You will eventually be able to create the perfect heart shape of any size you desire.

And remember: Nothing says “I love you“ more than making romantic smoke signals that will eventually kill you. Smoking is bad mmmkay?

NB: Does anyone know how to blow an arrow shape? The combination would be pretty cool, no? ;o)

Note for all you pedants: Yes I guess they should actually be called "Smoke Hearts" really but that doesn't sound as good, and a bit too darkly ironic - heart disease anyone?)