I attempted to do this with Slade this weekend. It would stay up when he was a puppy but I could just not get him in a mohawk this wknd. So I ended up cuttiing most of his top knot off out of frustration.
My mom's yorkie/chi has a natural mohawk. Her hair only grows to about 3 inches on top, but its really light and fluffy hair so I doesn't take much to get it to stick up.
I'm on my way to bed now, but if I get a chance tomorrow I'll post Slade and Schatzies 'hawks.