Samsung Galaxy S video and photo quality is pretty good but only when fully focused. The focus is getting lost very easly and it become blurred. Are there any ways to increase focus quality and speed?
I think you've run up against the limitation of the fact that its a multi function device that also includes a camera.
The camera focuses by contrast, so pointing at a particularly 'contrasty' portion of your subject may help. Additionally, in lower light an additional light, like a flashlight, may help (some users report that using a laser to aid focus works well).
As far as modifying the camera or additional software for the device to focus better, its currently unlikely.