Collaborations initially made up 80 percent of Leane's business, but today they're 20 percent, letting him to focus on his flourishing label. "The two complement each other. It's an experimental process, creating new pieces for the catwalk," said Leane. "You are constantly challenged in terms of weight and gravity. It can be really costly. And then you can still find it doesn't work."

For fashion eyewear maker Linda Farrow, special collaborations account for 60 percent of business. The company started working with designers in 2004, and now creates eyewear for Luella, Raf Simons, Eley Kishimoto, Matthew Williamson and Chloë Sevigny. For fall, a line with Antonio Berardi will bow.

The eyewear is made for runway and retail, and sold in designer outlets and at shops stocking Linda Farrow, like Colette, Browns and Selfridges. "You reach lots of different consumers. The Luella customer might be different from Williamson's. It extends your audience," said Farrow partner Tracy Sedino.

But most accessories designers say the biggest draw of these collaborations is the creative process, and the chance to work with the stars of the business. "Some things are a real labor of love," said Treacy, whose giant floral hats adorned the Valentino Couture runway in January. "You don't spend a week working day and night on Mr. Valentino's hats for the money. You do it because it's the pinnacle of what you do."

For fall, he's also doing pieces for Ralph Lauren, Donna Karan and Rifat Ozbek for Pollini. "Ultimately, we are all fashion animals," Treacy said. "We give up our services in the name of elegance and beauty. It's a total truth."