Probably the best sales will be in August. Keep an eye out for "tax-free week" when there is no sales tax on clothing and footwear (and items used to make and repair them) costing less than $110. (threshold for past exemptions) The website with more info is:

http://www.nyc.gov

I'm not sure when the next tax-free week is, I think they are usually twice per year. There is currently a proposal for the next tax-free week starting August 16th, but it isn't yet for sure. My last NYC visit happened to coincide with the January tax-free time and it was great for clothes shopping.

Also, don't forget to visit the "Century 21" store with bargains galore. It's directly across the street from the WTC site. Go early in the day and try to avoid the lunchtime crowds.