I used to use my phone, but for me my phones typically die/fail on me within a year, and some have poor battery life, or they develop it.
So for about 3 years I've owned a 16GB Sony Walkman MP3 player; it's battery can last me 2 riding days, it's fairly small (like 4th gen iPod nano size), and has a button lock slider to prevent ghosting. It's price point was it's main attraction ($110 new). So far it's held up great and does what I need it to do.
I always wear a helmet so in-ear earbuds are a must. Currently I have Shure se315s with dual drivers as I'm a bit of an audiophile. I like that they wrap around your ear so they can just hang when they're not in, but the the best part about them is that the actual earbuds are detachable. If the cord fails at any time I can simply replace it for half of the earbud's cost.
Word of advice: make sure to get earbuds with an L shaped jack rather than a straight jack. I've had many an earbud fail on me because my MP3 player typically ends up lying sideways in my pocket putting strain on the cord of a straight jack. This is probably why my Sony earbuds have lasted me 1+ years (which by the way I do recommend Sony's earbuds for good affordable earbuds).