Pixeldolls has some of the most beautiful clothes in Second Life. They have some GORGEOUS freebies available too. They are also having a big sale. They are working on moving from the ginormous store they have now into smaller boutique type stores. So run, fly or teleport there as fast as you can while these things are still available. The turquoise dress is the fantastic dress 'celeste' it comes in lots and lots of colors. The thing I love about it the most is the texturing. Even though it is a shirt, pants and prim skirt, it looks like a dress.
A word about skirts. Most items that are created using the actual second life skirt template (you'll recognize it from the purple skirt icon next to it in your inventory)otherwise known as a 'system skirt' tend to make out avatars asses look really big. There are designers who have found ways around this and they are genius. However, for the most part the best way to make a flattering skirt is to use the pants templace (glitch pants) and attach a prim to it that makes it look more tight fitting. You'll understand what I mean when you've tried a bunch of things on.
The other freebie pixel doll folder has lots and lots of really pretty tank tops, even a ballet leotard and a black shrug. It is perfectly acceptable, even recommended to mix and match different parts of different outfits and even different designers. It's called STYLE baby. In this picture I have put on the Wrap tank: dragon and then used the bottom half of the violet celeste dress as a skirt.
Celeste Dress: L$0
Wrap Tank; dragon: L$0