Was it right at the next junction, or left? You would check your map, but you're on a bike and the last time you attempted to read your phone while pedalling resulted in a smashed screen and bruised pride.
A far safer navigation solution looks to have been provided by smrtGRiPS - don't worry, the idea is much better than the spelling.
A clever upgrade on the traditional bike grip, the smrtGRiP sits inside your handbar, with a rubber outer keeping it in position. By connecting up to your smartphone via Bluetooth, the smrtGRiP provides screen-free navigation with Google Maps: having selected your route, the appropriate grip will vibrate to announce when to take the next left or right turn.
In addition to nudging you through city streets the grips can also connect to fitness apps, providing haptic feedback when you've hit your daily goals. A smart separation feature also lets you connect with up to 10 other riders, giving you a nudge if you find yourself pulling out ahead of the pack without having to turn around to check. Should your bike get pinched (or you just forget where you left it), an in-built GPS will help you down your beloved wheels.
Currently undergoing crowd-sourced funding on IndieGoGo, you can order a set of smrtGRiPS for £39. That, or stick with your dodgy TomTom-with-Duct-tape hack.