While many designers are still using clipping masks to render their flat sketches with fabrics, it's just not the most efficient way anymore. In CS2, Adobe Illustrator introduced the Live Paint feature, which is by far the fastest, easiest, and most flexible way to render garment sketches with patterns.
Live Paint will create separate fills for the different sections of your sketch (such as sleeves, collar etc) and they can each be rotated, scaled, or moved independently, without all those invisible clipping lines getting in the way!! And the best part is there's no need to draw your sketch as a closed shape, or join your anchor points!
Here's where I learned how to use Live Paint to fill my sketches with color and patterns:
Adobe Illustrator for Fashion Design
http://www.designersnexus.com/store/adobe-illustrator-for-fashion/
It's such a lifesaver, I totally swear by it!!