As funny as it sounds, after a long time of trying various techniques on a small touchscreen, I have found that I type the best by using just one finger :) That's after accounting for deleting typos/mis-hits that result from using multiple fingers. This effect is even more pronounced when writing on a moving vehicle. The device simply shakes too much for any accuracy.
Of course, this depends on the on-screen keyboard design on a given device. If the keyboard "buttons" are larger, it will be easier to write accurately. There are also some alternative typing methods, such as buttons that you press and then drag to get a given character within that button. Say, button 1 contains A, B, C, D, button 2 contains E, F, G, H, and so on. In theory, you can use multiple fingers more effectively and reliably, after some practice.