In order to achieve a really nice swirl you need a nice big star tip. I use Loyal brand, number 17.
I've only seen them in Australia.

I use French Butter cream, it's a secret recipe ; )

UPDATE 9th August 2008
I have been receiving lots of questions in regards to where to get the Loyal tip. I finally found a supplier online.

http://www.foodutensils.com.au/star-thermo-piping-tubes-p-1341.html