I found that the advice in animation books like The Animators Survival Kit and Drawn to Life was some of the best I've found for folds in a comic art context, which is largely to not draw too many of them, focus on folds that actually affect the outline or silhouette of the fabric and use folds mainly to show tension, bending, movement and relaxation.
Try to limit yourself to only a few folds if you can, and choose the ones that best help define the volume or show the movement.
The Etherington Brothers have some great tutorials about using folds to show movement and weight and stuff:
As well as the other important aspect which is defining volume. The sense of fabric wrapping around the shape of the body is a really key element:
Thinking about the body as having volume and the fabric as a thing wrapped around it is probably the biggest aspect to get your head around. It's kind of all about nailing that down and then working out the fewest and most impactful lines you can use to communicate it.
Common ones I use are:
- When arms are out or up, you'll often get a diagonal crease across the top of the bicep where the fabric is pulled taut.
- When the elbow or knee is bent, the fabric will bunch up in the crook of the bend. This also happens if the person bends their torso right over, or really on the inside of any intense bend, like if the shoulder is raised all the way up, or at the hip joint when sitting.
- There's often a crease on the chest if a garment isn't perfectly fitted there, usually either vertically between pecs or breasts or horisontally under them. Which one kind of depends on the looseness and the shape of the person's torso.
- Sleeves and pants legs are tubes, they might flop around and drag. Sometimes you can see the inside of the cuff.
- Trouser or shorts legs around the pelvis are really tricky, and the most important thing is to remember the volume that exists between the legs! When one leg is raised and the other is back, you'll get bunching at the hip joint of the raised leg, no bunching down the lowered leg because the fabric is in tension down the front of that leg, and you'll also get tension in the line running between the legs. You might even get a crease under the bum on the back leg.
I think you can see practically all of these in this panel from chapter 3:
