How do I know when my iPad needs replacing?

I'm curious, how can I tell when it's time to upgrade my iPad? Is there a specific set of signs I should be looking out for, or is it more of a personal preference? Are there any key features or changes in technology that indicate a new iPad might be worth the investment? And, if I do decide to replace it, what's the best way to go about transferring all my data and apps to the new device?