The app is designed to save your login information on your phone like most other apps. If you find yourself logged out don't panic and don't create a new account if you've already used the app.
If you find yourself logged out often this is predominantly caused by a lack of space on the phone. Even if you are not getting warnings about the lack of space the phone could be determining that the files storing your login information are less important and will remove them on their own. I have no control over this process. The only thing I can suggest is try and clear some space on your phone and see if ServerLife starts keeping you logged in.
If you are having this issue please contact me in the chat so we can troubleshoot together.