I have to do this quite often. I end up using a scanned image of my signature with Gaussian noise added in Photoshop. You can add Gaussian monocromatic noise to

I have to do this quite often. I end up using a scanned image of my signature with Gaussian noise added in Photoshop. You can add Gaussian monocromatic noise to the whole page (and rotate it slightly, for good measure) to make it look like you killed a tree.