You can get this conversion kit for canti/V-brake mount 135mm rear ends http://2btherapy.com/bikes/brake-therapy-conversion-kit and install a disc compatible front fork. No solution if you are currently on caliper brakes or narrower frame.

By the time you have bought all the necessary stuff (and it mounts up) balance of investment might favour a 'new to you' bike that already has discs, so best to do the sums first.

Have to ask; driven by fashion (nothing wrong with that) or driven by braking performance? There is a weight premium...