Had P3S for a month and a half by now.  Didn't have any levelling problem initially, but noticed that the horizon gets tilted by about 10 degree about a week ago.  Searched the forum and performed IMU and gimbal calibrations, which didn't help. I then noticed that one of the four vibration dampers got disconnected from the Camera Vibration Absorbing Board.  Plugged it back in and the problem solved!

So if u have the tilted horizon problem, it is worth to check if u also got loose or broken dampers.

you can also correct it in the gimbal control in the Go App.   

Good advice, Thanks! DJI recommends a preflight check that should always include looking over your airframe for damage -> http://www.youtube.com/embed/FgFxsaIUvbQ
In fact, check out all of the DJI videos for your model, if you have done so yet: DJI P3 Std Video Tutorials
If you search various forums, there are multitudes of preflight checklist such as this one for the P3P/P3A: UAV Flight Log or perhaps search PhantomPilots Forum

Another source for preflight checklists is P3 Ground School

These can be customized into your own checklist since flight planning is a critical part of safe aviation that is often overlooked by the beginning pilot.
A simple Android app called Simplest Checklist allows you to create your own checklist, modify and order the steps, and gives an easy way to ensure that nothing is overlooked or forgotten as you become more familiar with flying and begin to cut corners as we all do.
The Apple App Store has an endless assortment of both free and paid similar apps such as Paperless Lte: Lists + Checklists or Clear. Search and find one you like!

Old post but here's what I did.
First, I don't put it in the fridge just the fact someone mentions "condensation" on the camera tells you that's going with the electronics too.
water + electronics = dismay

1: Level surface,  turn it all on, controller and drone.
2: Turn on grid for camera (you will see grid lines in your camera view) these are too help orient you.
3: Do IMU calibration.
4: When it's done go to gimbal roll adjust (because IMU calibration alone does not fix it) adjust the gimbal roll so that the grid lines in your camera are horizontal and match something level in the view.
5: Do gimbal calibration.
6: Done, at least for me, the camera now stays level.

Your mileage may vary, but keep at it and you will find that one of the combinations of things here will fix it.