Also work on form. It's everything on high-rep sets, but it really increases the benefit of pullups overall, too.
Grip the bar hard.
Get a full stretch at the bottom, pause for a split-second, then explode up. This is the most important part, I can't emphasize it enough. Pull as hard as you can, not just hard enough to barely get you to the top.
Make sure your chin or even your collarbone is at the level of the bar...
Then drop into the eccentric. I see no reason to emphasize the eccentric portion of a pullup, personally, and you still have to decelerate at the bottom, which is arguably better than a slow eccentric.
You can do more pullups this way, and if you do them faster you are exerting more force which equals more strength gain.
Oh, don't ever raise your legs. That's cheating.