The best approach I found is in Apple’s Technical Note TN2262: Preparing Your Web Content for iPad.
- Test your website on iPad, and update user agent detection code if necessary
- Use W3C standard web technologies instead of plug-ins
- Check your viewport tag settings
- Modify code that relies on CSS fixed positioning
- Prepare for a touch interface
- Use textareas instead of contenteditable elements
The user agent thing under the Simulating Safari on iPad HTTP requests in Safari on the desktop was really helpful.