OK this is a little tricky, Adobe never implemented in Photoshop a tool to draw symmetrically in real-time, thing that programs like Artrage, Painter, Alchemy, etc. already include since... well, years, but there's a way to approach that idea using a mirrored Smart object, it isn't real-time at all because you must to hit Ctrl + S to update the preview Window but it's closer!
If you want a little help, I'm sharing a Photoshop Action that make the hard work for you (please, read the instructions inside the ZIP), right here:
UPDATE: Actually you can do it a lot easier using AD Brutus Symmetry, a Photoshop Panel I created some time ago. It includes this same technique (Smart Objects) but also a better way to create symmetry using vertical and horizontal Guides. Watch the promo:
Thanks to John Nack (Adobe's Principal Product Manager) for the article about this technique (Symmetrical drawing in Photoshop)