I am also really flatfooted with narrow feet. I wear the Dr. Schoell's (spelling?) inserts for work and hiking. There are probably better ones out there, but those ones are readily available and can be changed out regularly. The arch support ones move around too much for me and I get hot spots, so I just use the soft pad kind. The pads do take up some room in the boot, but mostly it seems in the length area. The wool socks that I don't wear other than hiking or hunting seem to help in the width area the most. What seems to help me the most is good ankle support. My ankles aren't great because of the flat feet, so lacing them tight and sometimes even an ankle brace occasionally to help out because the boots seem to fold over when they get broken in because of the flat feet problem. Don't laugh but I have been doing yoga lately and I am surprised at how it stretches out my feet and the arches actually seem stronger. I haven't tried any liners, though. It might be something for me to look into, too. I am not sure that helped at all other than having someone else with flat feet on here.
