Here are a load of things that I often want to look up when using the xy.sty LaTeX package.

• Centring the label on the arrow: the - in \ar[r]^-{f}
• Invisible arrow: the @{} in \ar@{}[r]|{f} (the | puts the label in the middle of the arrow rather than above or below it).
• Shifting an arrow: the @<> in \ar@<1ex>[dr]