I forget the original source of this tip, I learned it years ago and I wish I could give credit where it's due.
Want to lock your iPhone? Four characters is better than nothing, but it's not great, since most people tend to think in predictable ways. But turning off Simple Passcode gives you the keyboard and makes it hard to enter quickly.
Turn off "Simple Passcode" and enter a passcode of arbitrary length that is only numbers. When iOS shows you the unlock screen, it will show the number keypad and allow unlimited input. From now on, you enter your passcode and hit OK to enter your code. Almost as easy to enter, much harder to guess.
