Men: Appropriate for Every Occasion
Casual: Let's say the event is a lunch at a casual eatery with a few friends. Firstly, you can identify a casual eatery because while someone bring you your food, you won't be expected to be dressed to the nines. Something above Denny's but below the fancy places. This one couldn't be easier, right? Well, maybe. Logo t-shirts and your broken in jeans are out. Pick a nice pair of jeans or khakis and a nice shirt, preferably a polo or button-down. That may seem dressy to you more casual types, but a good general rule is to shoot higher rather than lower. If you underdress you'll look sloppy. If you overdress you'll still look good, albeit better than everyone else. But hey, that's a good thing.
Semi-Formal: Now it's time to start stepping it up. A good example of this might be an evening of dinner and dancing on the town, or a holiday party. A nice pair of slacks is a must! Khakis won't do it anymore, unless you're going for the khaki-navy blazer combo, but quite frankly that look is as old as The Love Boat, and deserved to be left there. Pair your trousers with a crisp, clean button down and, if you feel the occasion calls for it, a tie. I'd say on these occasions it's okay to leave the jacket at home, but again, be the judge of the situation for yourself. Also, it's easier to take something off than put it on, so if you're not sure, bring it with you. You can always leave it in the car.
Formal: This is a more special occasion, like the rehearsal dinner before a wedding, or seeing the philharmonic. Now the jacket and tie are requirements. And just because I didn't say so before, if you're wearing nice slacks or a suit (as would be perfect for this occasion), make sure your shoes are just as dressy as the rest of your outfit. Nothing ruins a look like incongruency.
Black Tie: You've graduated to a fancy black tie event, eh? You'll need to rent a tux. Think the prom, or a night wedding. There are tons of great tuxedo options nowadays, so you won't look like every other penguin in the room, unless you want to, of course. Fortunately for you, black tie events are usually well-marked as such. I've never seen an invitation to a black-tie event that didn't explicitly state "black tie," so this one is easy for you!
Now get out there and show your date your dressing expertise!