First off, you have to know when they are going to be happening! To know when Aurora is active sign up to get notifications from www.aurorawatch.ca
When you get a notification of a red alert you know it is time to get your gear together and head out! Don't jump the gun though, as sometimes it peaks for just a moment early in the night and then nothing after that. Wait until the alert stays for two or three bars and head out around midnight. The best viewings tend to be from 1:00-2:00 A.M. There can be sightings anytime of year but during the summer months they tend to be more active.
Now for camera settings! Northern Lights are extremely difficult to shoot and one of the few times where it does help to have great gear as well as know what you are doing! If your camera can handle higher ISO's then try 1600-2500 ISO, if you have an older camera you might want to stick to ISO 800 or risk the photos being very noisy. Next, you want to use as large an aperture as possible to gather as much light as possible, if you have a wide angle lens with 2.8 or faster then perfect! If you've got the kit lens you might be struggling with using 3.5 as your aperture. For really spectacular Northern Lights a fast fisheye would be amazing! Aurora likes to dance, which means the lights move fairly quickly. A shutter speed of 2-4 seconds will allow your camera to capture enough light without loosing the amazing patterns they make in the sky. Using 4 seconds for your shutter speed might be difficult if you have had to choose ISO 800 and an aperture of F3.5, then you can have a longer shutter speed to get the colors of the lights, but you will lose the pretty streaks. Make sure to use a tripod, no one can hold still for four seconds.