Hi Miranda,
Sometimes if I need a little extra smidge of storage space, I will use the very tiniest of little underseat bags. Except, I attach it right in front of my head tube.
The velcro strap that would normally go around the seat tube...I put around the head tube instead. And, to give a little support from the top, I use one of those tiny, tiny little bungee cords (about 3" long and very skinny) up to the stem.
I will stuff into this little bag the stuff I hope I won't need (a couple first aid swabs, a packet of ibuprofen, a tube and cartridge). Doing that frees up just enough space in the "real" underseat bag to give me room for something else.
If I am going out for an absolutely all day, solo, unsupported ride, I will add to that a little Bento Box for the top tube. I will stash a few gels or bars in there...also creating just that smidge more room in the underseat bag.
I also hate having my pockets jammed full.
"The best rides are the ones where you bite off much more than you can chew, and live through it." ~ Doug Bradbury