When reading email using mail, I always click on a link and it open in safari. But when I want to go back to mail, I need to press the button and then find email. Is there a better way to seamlessly switch back to mail, with a gesture or something?
You can use multi-touch gestures to swipe to the last app you used, in this case will be mail.
First make sure Multi-Touch gestures are turned on in the settings panel.
Once this feature is on, you can swipe from right to left with four fingers or more to cycle through the apps you last used.
If you do not want to use that, you can double tap the home button top bring up the last few apps used.
As far as I know there isn't a gesture: but you don't have to find email again. If you double click on the button the lower bar will show all the (open) applications in order of recent usage. Mail should be the first one.