A wheelchair using guy on one of my flights had a medium size duffle bag that he somehow managed to sling under the chair. I'm not sure how he did it - he may have had
an under chair catch all. No center of gravity problems for him.
I guess my helpful hint would be to try to consolidate things as much as possible so that you don't have to keep track of a million separate pieces. Like, I have a small bag that is my purse-equivalent that lives under my chair; when I'm flying I make sure there's enough space in my backpack to stuff it in when I board the plane so I don't have to worry about three things (backpack, under-chair bag, cushion), only two. Bags that snap or strap together are good.