Have a long list of songs or apps? Tap the letter block (I.e. the square with 'P' on it) and jump through long lists!! This also works with any app that uses the letter blocks
If you find your phone's battery life isn't all that great, go to settings > system and then background tasks. Deselect any apps you don't want running in the background
Hold down the full-stop button on the keyboard and get additional punctuation!! Similarly this works on a bunch of other keys (e.g. holding down 0 gives you the degrees sign and holding down 1 gives you fractions)
Holding down the numbers key and sliding your finger to the desired punctuation/number inputs the character and returns you to the main keyboard.
Have a preferred Camera app? You can set which Camera app you want to be launched when you hold down the camera button - just go to settings and then photos+camera and scroll down to default Camera app
Can't find a menu or setting? Everything in WP is achieved by the three dot menu (usually bottom right corner) or by tapping and holding on an item.
Pin small-sized tiles of your frequently used contacts to Start for quick access
Album and artist art not showing properly in music+videos? Use MPATool from the Windows Store
On the now playing screen for music+videos, swipe across on the album art to change tracks
To instantly switch your ringer volume from on to silent, press the volume button once and tap on the bell icon in the volume HUD. This also works in reverse
Use Data Sense to keep track of your data usage and see which apps are hogging the most data on WiFi and cellular.