I'm a believer that- if it doesn't fit in your jersey pockets- you don't need it.

I don't use a seat bag, they always rub on my shorts- no matter what..
tube and co2 go in a ziplock bag in my middle pocket, phone and house key in a side pocket and then gels, blocks,etc. in the other pocket. I will carry my debit card with me so I can stop at a convenience store if needed.