Rule 1:
Know your dress
That's right have a clear idea of what your dress is like and, most importantly, made of. This'll make is so so much easier to shaded and draw, for example:
See the above picture (No?!) well I've (tried to) shaded a dress three different ways to try to give 3 differetn material effects. The first is knida like silk (a lot of imagination needed here people) notice how with *ah hem* silk the shading is at a high contrast. Use hard brushes only and keep working into the dress with light and dark tones. Also you can see hints of a kinda purple colour - that's my secondary lightsource showing. Don't actually pick a purple it's just a grey tone of my red but shows up as purple. Another hint for silk is to use the smudge tool (90-100% strength) and a hard brush. It can make adding in folds a lot easier!
The second is, well even I don't know! But it is textured in a kinda woolen way (?) anyway the important thing is to notice how there's not much difference between the highlight colour and the shaow colours. I used a bigish brush that was a hard brush but just ste it at a lower opacity than the silken dress. I then added a textured brush over the top but you can create the same effect by making a hard brush set to scatter.
The third dress is...greyer. Hmm I think I was thinking of those prom dresses you get that look like that. But the important thing to notice is that this was made using a hard brush at a low opacity and lots and lots of layer of it aswell. Greyer tones were used instead of white highlights as the material is not shiny. As the material is not heavy it doesn't create lots of folds either.
I hope you can see where I'm getting at here! Experiment and use the internet to find pictures of different materials and study how each material creates folds and how it catches the light. Add varietry to your dolls ^^
Step 2:
Folds to the rescue!
Ah folds, the devil's play thing. That is folds right? Well anyway they're a pain! Knowing where to put folds, how many, which direction, how thick etc. is a full time job and will make you envy films with big dresses where they don't have to pick out the folds they just let it fall. Right?! I'm going off track once again arn't I? Okay on with explaining the gif! Ah gif... the devil's pla-
In the first image I had a clear idea that the dress would have heavy maerial and fall down in straight lines. You can see how the folds at the bottom have a minimum amount of curviness to it but when it does it curves like the top of your head (like this way ^ not v ). When the folds do not overlap a small curve is produced like the red line on the bottom left drawing. That's about it really. Keep folds straightish and not curved is the main message. Use a bigish brush.
The second dress has lots and lots of folds! I made a squiggly line to start with the drew lines upwards to help me see where folds should go. The surves have sharp bottoms which curve towards the floor in contrast to the first. The dress sides curve also towards the hips. You can really have fun with these dreses and add folds and detail a plenty- use a small hard brush with a high opacity. On the top image you can see I added curves like j's which are where the material has hit the floor and slightly been bunched in on itself. Go nuts.
The third dress has soft curves on the floor and you can even add flaps where the material overlaps. This uses a higher contrast of colours than compared witht he first dress because there are more folds. I used a brush similar to the 2nd dress - small, hard, 100% opacity. This material really flows so try to encorporate that! If there was a slightest bump (Hint: knee) the material would pick up on it and flow from that point as well as the hips.