Keep it short.  Make your point succinctly so most of the text and fields appear above the fold.

Functional and attractive design.  The design should make it clear what the action is and what the next steps are.  A petition is a great place to have some fun with design, too.
 
Time limit (if applicable).  If you need X number of signatures by a particular date, then make it clear what the deadline is to add a sense of urgency.

Numerical goal that you display on the petition page.  It helps to know what the goal is - people get excited when they see the thermometer inching upward.  Remember Howard Dean and the bat?

Tell a friend.  After someone signs the petition, they should land on a Tell a Friend page.  This can increase viral activity by a factor of 10.  Don't make them click again if you can help it!

Don't neglect the autoresponder.  Make sure that the automatic e-mail message people receive after taking action contains a thank you and some additional actions (Tell a Friend about this petition, take action on another advocacy campaign, or make a donation are great ones).  You have their attention, and they probably feel good about signing the petition, so use it!

Here are just a few of my favorite petitions:

www.MeetTheRealMcCain.org from NARAL Pro-Choice America - very clean design

www.millionformarriage.org from Human Rights Campaign - an ambitious numerical goal that updates in real time (Disclosure:  I worked at HRC when this petition was launched.)

www.petitionforpoultry.org from the Humane Society of the United States - a VERY fun thermometer image