One method frequently used by Volvo mechanics is to pad the jaws of a big set of Channel-lock pliers, open wide enough to catch the edge of the injector tip and some of the plastic collar, then squeeze carefully. If you want to save the collars, it would be wise to scrape or wire brush off any crud on the inector stem below the plastic, and hit it with spray lube as others have mentioned. Once you get the o-rings and plastic off, you can clean the outside of the stem lightly on a wire wheel, making lubed reassembly that much easier.

George