For me, the perfect solution to fit (comfort and having it stay in my ear) has been to use a Jabra EarGel. I got them from Amazon, with each package containing three pair (small, medium and large)...
http://www.amazon.com/gp/product/B00004WINT/103-0456661-0716623
The fit of the eargel on the Apple Bluetooth Headset is tight, so work it onto the headset carefully. But once it's in place, it will stay there with reasonable care inserting and removing it from your ear. Of course, if you just grab the boom of the headset and rip it out of your ear, the EarGel is gonna remain in your ear. Just gently pull the EarGel and it'll come out with the headset attached.
I
did first try a Griffin EarJam, but it stuck way out from my head, and in no way followed the line of my jaw (i.e., it kind of stuck out into space). Not to mention the fact that the EarJam is white, so it looked ridiculous. But the Jabra EarGel I am using is clear, and I end up with the absolutely perfect positioning.
I know nobody wants to spend more money after the cost of the headset itself, but the EarGel pak is $9.00 and pretty much a no brainer.
I couldn't be more pleased... 🙂