The life span of a bottle of cologne is one year. Even if you haven't gone through the whole thing, after twelve months buy a fresh one; you don't want to wear a stale fragrance.
Test in increments
When shopping for a fragrance, try two at a time at the most, one on each arm. Sniff your shirtsleeve between tests to clear your palate.
Keep it simple
Leave spraying wrists and other pulse points to the ladies. One hit on each side of your neck will do.
Like your wardrobe, your scent should change with the seasons. In the warmer months, wear a small amount of a lighter, fresher scent; in winter, heavier scents like musks in slightly larger doses are appropriate.
[courtesy of GQ -- we like when someone else says exactly what we wanted to say!]