There's probably a better way, but here's how I just create the below nikesque swoosh:
1) use the pen tool and create the bottom line of your swoosh, the pen/bexier tool is weird, basically click, move the mouse to the second point then click and drag to get the desired curve.
2) point one will give you a layer mask, rasterize this.
3) now create the upper line of the swoosh on a new layer using a different colour.
4) rasterize the upper layer, merge down and use the magic wand to select the odd block created in point 3)
5) touch up any bad bits by hand
NB: using Photoshop 7.0 btw