2) Taste everything.
A great chef would never serve anything without knowing that it tastes good, and neither should you. How would you know if an ingredient was forgotten or has spoiled? How would you know if you under- or over-poured something? Furthermore, tasting it all - including drinks you didn't make - gives you knowledge and perspective on your bar program. Tasting cocktails made by other bartenders also helps you figure out your personal progression. Does the experienced bar manager's Manhattan taste more balanced than yours? Ask him or her why. This applies to wines and beers as well - if you are the opening bartender, taste the tapped beers and open wines to see if anything is oxidized or flat.
Your palate will tire after a night of tasting, especially after eating or drinking anything with a strong flavour (like espresso or Fernet, for example). A great way to keep your mouth fresh is to sip soda water throughout the night. The acidity will cleanse your palate, but don't drink flavoured soda as the sugar will dull it. Most importantly, if something tastes off and you're not sure why or how to fix it, get a second opinion. Ask another bartender or a knowledgeable floor manager to taste your drink to see if it's servable. You will occasionally have moments of only being able to discern taste as right or wrong, so don’t be afraid to ask for another opinion.