Summer is warm but can quickly turn cold. Bring shorts and t-shirts for the heat, but also pants and jackets for the cooler days and nights. A fleece is basically mandatory. So are comfortable walking shoes, as you will be doing a lot of walking. Also, don't forget a hat: the sun hits hard out here.
Winter is cold but chinook winds turn many days into above-zero temperatures without warning. Bring all of your winter gear, of course, but make sure it includes some lighter jackets; you don't want to get stuck in +10 weather with only a heavy down on hand. A fleece is basically mandatory. Winter boots will likely come in handy, but bring walking shoes too: chinooks melt the snow in the city so that roads and sidewalks, for most of the winter, have no snow on them.
And most importantly, whatever season you come in: bring a camera!
