Flat horizons. You can use the 'measure' tool to work out how much to rotate a picture to make any line either perfectly vertical or perfectly horizontal.
Rotating in photoshop is great to correct tilt that went unnoticed when the picture was taken. Another help is to use a grid on the LCD or viewfinder. I know some film cameras (mainly Nikon) have this feature in the viewfinder. I don't know about the DSLRs.
Awesome tip. I never even thought about getting lines perfectly horizontal/vertical until I submitted my first picture here. Such a small detail that makes such a huge difference.
