Wear what you feel comfortable in
The most important thing to keep in mind when dressing for a date is to wear what you're comfortable in. There's no point in wearing four-inch stilettos if you're hardly able to stand, much less walk in them. The same thing applies to the guys as well; don't get all suited up if you hate wearing suits.
Dressing nicely doesn't mean you can't be relaxed and comfortable. The main reason you dress up a little (or a lot, depending on the date itself) is so your date will know and appreciate that you've made the effort for him/her. Another good reason is so that you don't appear to be a slob. Its just not a good look. This includes the seemingly ever-present thong/g-string above jeans/trousers. I don't care that celebrities do it. They too get panned by the gossip magazines when they appear in public in such a manner so you know its just not the way to go.
If you can, always find out where the meeting will take place. The majority of places will be alright with just jeans/trousers for both men and women or even a casual skirt/dress for the women. If you do plan on meeting at a slightly dressier venue, a long-sleeved collared shirt is a safe bet with jeans or trousers for men and the women should be able to get away with a skirt/trousers with a pretty top or a dress with good accessories. Fashion today has come a long way so even jeans can be worn to dressier affairs. Just dress them up a little with a glam or sparkly top, killer shoes, a great handbag and accessories.
Don't over-do it
Over-dressing is as bad as under-dressing, if not worse. With men, its harder to over-dress than with women. I doubt there are many men ho would willingly always wear a three-piece suit when going on a date, unless its a business meeting (and definitely not date-worthy) or a very formal show/dinner. With women however, there is a tendency to go overboard with clothes, accessories and makeup. Unless you're off to the casino or a Mardi Gras, is a safe bet that your gorgeous sequin cocktail number is out of the question. especially when you decide to pair it off with earrings, bracelets, rings and necklaces. For something that glitzy, stick to a simple pair of earrings and maybe a bracelet/bangle. The same goes when you're wearing a sparkly top. you want your date to see you, not to be overwhelmed by all that glare.
Letting it all hang out
To sum up that statement in one word; DON'T. I know when the weather's nice and warm that its all too tempting to wear that thigh-skimming mini with an itsy-bitsy top and skyscraper heels. But please refrain from doing so. All that outfit screams is "easy". And that's not what you ant your date to think. Unless of course it is, in which case you may as well ignore this paragraph. When it comes to legs, breasts, stomach and back, stick to just showing off one, or at the most two, of these. Any more than that is over-kill. If you have a date at the beach then this rule is moot point. As for the men, the buttons are on your shirt for a reason, the ladies don't want to see halfway down to your belly-button. Neither do they want a view of your entire chest and stomach through a transparent shirt.
Too much makeup
This one applies mostly to the ladies. You know how you're shopping and you find this amazing dress/top/skirt/etc. and it's got streaks of makeup from someone else who had previously tried it on? Now think about how the men feel when they find streaks of foundation on their shirts. okay, so you can wash it off easily. But that's not the point. The idea that the girl had been wearing that much makeup is a turn off for most guys. Not to mention how the lady in question actually looks when her face is caked in such a fashion. Yes, it is noticeable. But other than not using so much foundation and powder, you have to also keep the rest of it simple. You either highlight your eyes or your lips, not both. The whole Marilyn Manson/drag queen look will not win you any favours.
Wear the right shoes
You'd think this is an obvious one but surprisingly, it isn't. I don't just mean the right shoe for the occasion, though that is very important. I mean the shoe that suits your shape and what you're wearing. For the men, you can generally get away with a loafer or sneakers if you're wearing jeans. If you've got on trousers, just stick to a pair of loafers or even dressier shoes if you're somewhere a little more up-market. Day-time footwear can be more casual, try a nice pair of sandals or flip-flops. Leather ones look especially good with light-coloured cotton or linen trousers. Women have a wider choice when it comes to shoes. First of all, wear shoes that suit your figure. If you have slighter shorter legs, don't go wearing sandals with ankle-straps. They will only 'cut' your legs, making them look a lot shorter and chunkier than they are. The same applies to wrap-around sandal-laces. This sort of shape also looks best with a shoe that has a bit of a heel. Even if you're off to the beach, choose one those thongs with the slight kitten heel. Its low enough to be practical but still flattering. For the taller women, you can get away with nearly everything. Just keep in mind that your centre of gravity is a lot higher up with high heels. As for the occasions, save the bejewelled sandals for a night out on the town. If you're a bit of a 'Carrie' dresser then carefully pair you girly and sparkly sandals with a more laid back day outfit. It can work but just keep in mind that the day time isn't when you should go over-the-top. It goes without saying that you don't want too much height in your shoes if you're going to the beach or if you're going to be walking a lot. Painful feet do not, a great date make. And no grotty thongs or sneakers when you're dressed to party. You can never look right if your shoes are all wrong.
Have a good posture
I know this may have nothing to do with the clothes you wear but it is important to always have a good posture. Stand up straight with your shoulders back, tummy and butt tucked in. Not only is this good for you from a health perspective, it also makes you look taller, slimmer and more confident. Plus your clothes will also look a lot better on you.