How do I speed up a USB external hard drive?
There's not much you can do to "optimize" your transfer rate. OS X should be maxing out your USB connection as it is.

If that's not fast enough you might want to look at a USB 3 drive (if you're not already using one) or a Firewire drive which will give you better throughput. FireWire enclosures are a bit more expensive but they're worth it if you need higher throughput (for say, video rendering, etc).