Bicycles don't have a lot of ways to tell everyone on the road what you're doing. Instructables user abikerider fixes part of that problem with a DIY brake lever light switch that informs drivers you're slowing down.The nice thing about abikerider's method is that it wires through your existing brake lever in a clean way. It's not mounted on the handlebars or added awkwardly to the frame. Instead, you install a small push button onto the brake lever itself and then line the wire leading to the brake along your brake line. This way, when you pull the brake lever, the push button is engaged and the brake lights turns on. Take a look at the Instructables post for a full guide.
Add a brake light switch to a bicycle brake lever [Instructables]