I clean them along with the wheels. Use a stiff brush and dilute APC to scrub the sidewalls. When dry, use a tyre dressing. I use megs endurance tyre gel from halfrauds. The difference is that I apply using a brush so it gets into the lettering etc and I leave it to dry rather than buff off. Leaves a shiny wet finish which I love. If you plan on driving the car soon after, buff off the product. Poorboys natural look dressing is also good to use on tyres.
Remember, unlike showroom cars which aren't driven, the dressing wear off over the next few days.