Load the bike, not you.
Storing items in your jersey pockets is a good solution, but it can often lead to irritation. If you take a look at adventure cyclists, you'll notice the pack as much on their bikes as possible. Use the Louis Garneau Middle X-Race Bag to store stuff you'd normally carry to ensure your back is able to ventilate properly and isn't rubbing the wrong way while riding.
The Middle X-Race has a liter of volume, perfect for a tube, patch kit, keys, or electronics. Its zip closure is sealed to protect from road spray, and the pack is expandable if you need extra room. It uses a mirco-adjustable rail system that fits standard saddles and seatposts.
A hook-and-loop retention system offers bag stability so it's not bouncing around when you're sprinting. A rear light tab lets you easily add low-light safety -- and reflective elements are there too if you don’t have a light.