From a racing perspective, my jerseys have 3 pockets in the back and since I only have one bike to train & race on I don't use a saddle bag. I just put anything I bring in the pockets. In the left I usually have keys and gel or cliff bloks. center is flat kit in a bag wrapped tightly and bound with a rubber band. right is cell phone. If I bring a vest or take off arm warmers then I just combine keys, gel, and phone into one pocket and put the extra stuff in the other.

Hope that helps