While earbuds will fit, I've found something better...
Shamless Product Plug
I purchased a set of these about a year ago for $60 Canadian, and while somewhere hesitant about buying them at the time, I am tremendously happy with my purchase. Imagine if you will soft foam earplugs with built in speakers and a lightweight cord. That's basically what these are, you get 3 different sizes of shaped plastic plugs for your basic headphones, that will allow them to fit comfortably inside your ear.
They have some of the best sound quality I've ever heard from an earbud as the design allows the sound to be directed straight at the eardrum rather then just the general vicinity of your ear. I'm able to pick up beats and notes I had no idea actually existed in my songs. I love these, and would buy them again.
It seems like this sort of design would be very uncomfortable, but there have been times sitting on the bus to/from school (in the winter) that I have forgot they were in and wondered where the music was coming from for a couple seconds... you don't even notice they are there.
Oh, and since they don't stick out past your ear they sit right inside your helmet with no problem. It should be noted though, that I will only ever use them on the highway (no city driving) and I only use them in one ear, so I can still hear what is going on around me.
Hook it up to my mp3 player in my inside coat pocket and I've got 14 hours of music (with a full battery charge), though I'm carryin over 200 hours worth

Of course I'm generalizing from a single example here, but everyone does that. At least I do.
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