Using Touch ID is very handy, but not everyone has it yet. If you have any iPad, or an iPhone that's not a 5S/6/6+, putting your thumb on the home button and waiting won't unlock these devices. Keeping your data secure is still important, even if you can't tie it to fingerprint information.
In fact, even if you're using Touch ID, the quality of your passcode matters because your passcode will still unlock your iPhone. Using a passcode is the best way to keep everybody else's grubby mitts off your secret texts and high scores, and using a "Complex Passcode"—one that is longer than the default 4-digit "Simple Passcode"—is all the better.
Let's start with that default Simple Passcode. A quick web search shows the bad guys can crack a four digit code in less than 24 hours (if they have unlimited tries). Five, six, or even longer codes drastically increase the amount of time required to crack them.
But a longer code usually means fiddling around with a full keyboard every time you want to unlock your phone, and many folks won't want to do that. Here's the trick: if you set up your passcode using all numbers, even with that longer passcode, you only get a numeric keypad when you go to enter your passcode. That's a lot easier to navigate.
Here's how to set up a "Complex Passcode" in iOS 8:
Go to Settings > Touch ID & Passcode, and turn off the "Simple Passcode" option. This requires you to enter your current password again, even if you just entered it to access the Touch ID & Passcode. If you're device doesn't have Touch ID, go to Settings > Passcode, as shown below.
Settings > Passcode
Once you enter your old passcode, you'll be presented with a full keyboard, with letters on one screen and numbers and symbols on a second screen. iOS 8 is intelligent about this, though, so if you enter a numeric-only "complex" passcode, you'll only get the phone-dialer keypad when you want to unlock your iPhone.
While you're setting it up, however, patiently enter your numeric passcode twice using the top row of the second screen of your keyboard. If you're using a keyboard replacement app, use it as you normally would. Enter your new, longer passcode, and tap "Next" in the upper right hand corner of your screen.
This is different from the Simple Passcode setting, where it automatically advances when you enter the 4th digit. That's because your iPhone has no idea how long of a Complex Passcode you're entering. You must tell it when you are done. You'll also have to tap an "OK" button on your lockscreen, as shown below. This is so you don't unintentionally inform a would-be snooper how many characters are in your new and improved Complex Passcode.
More Digits, but Same Numeric Keypad
Note the "OK" Button Next to the Entry Field
In other words, enabling this feature will mean you have more steps to take when managing your iPhone. If you're concerned about security, it's worth it. If not, you may want to stick with the Simple Passcode.
Also note that the Touch ID & Passcode screen we're on is where you adjust what's available in the lock screen settings, and whether or not to erase all the data on the device after 10 failed attempts to enter the passcode.
Another important feature located here is the time limit on requiring a passcode. You can set it to 1/5/15 minutes, or 1 or 4 hours. This is the amount of time the device is asleep before the passcode is required again. For a lot of people, five or fifteen minute options strike a nice balance between security and convenience.
Now when you need to enter your passcode you'll see a numeric keypad show up, as if you're making a phone call.