There's at least 2 things wrong with plonter's post. Some IEMs come with triple flange tips and triple flange tips kill my ears.
The only in ear I own that I can wear consistently is Westone 1 because it doesn't go very far in your ear. I have to use large Shure olives to get a seal. I can use my Klipsch X5 only a few months out of the year because of allergies. If my allergies are acting up, they're just to painful, even with small Comply tips.
I've also had the problem with IEMs falling out when the wires just hang. If you can get the wire over your ear, they'll generally stay.
BTW, customs don't necessarily last a life time. If your weight changes by 30 pounds, they probably won't fit any more. My wife is deaf and it's happened to a few of her friends with their hearing aids.