We've talked before about using a longer passcode on your iPhone instead of a four-digit pin, but as tech blog Digital Inspiration points out, adding in accented characters adds an extra level of security.
The idea is that most people aren't going to bother dealing with accented characters (if you hold down on a letter the available accented characters show up) when they're trying to guess your password. To use these, you first have to turn on the alphanumeric passcode. Just head into Settings > General > Passcode Lock and turn off Simple Passcode. You'll be asked to enter in a new password, so throw in a few accented characters. It might make it a bit of a pain to enter in your passcode, but it will be more secure.
Create an Un-guessable Passcode for your iOS Devices [Digital Inspiration]