Quote CrystalNeko:
I look like a pack mule XD
This.
I have gone so beyond giving up on keeping things small, since I often have to bring costumes for our entire cosplay group. So on an average con I will bring my enormous suitcase (just for costumes & snack food), a backpack (just for make-up & toiletries), one of those big Disney Store re-usable bags (for props), often a weekend bag slung over my shoulder (with civvies if I remember them).
If I have big props that don't divide easily like Mini's Hidan scythe or my wings, I take them in a guitar case, since that way people are more careful with them, and it keeps them safe pretty well.
So yeah, I look like a mad woman getting through London, but I rarely forget things and it means that when all our group members come in to my hotel room asking "have you got a...", I generally have it, and a spare. It's no wonder some of them call me mum.
Only downside I find is that getting through turnstiles in tube stations is a nightmare, and train travel can be a permanent stress-out. Sometimes it is well worth the taxi fare to get back to Euston (cost around £10 each for 3 of us sharing a taxi before).
Thankfully I live 10 minutes travel from both Ayacon and Kitacon, so no need to worry about that.