When it comes to music, by far the best accessory I have for my iPhone is a pair of Etymotic earphones
with ipod controls on them.
Having those three little buttons up near your ears makes all the difference. One button is play/pause (and answer/hang-up if a phone call is coming through, or hold it down and then talk to Siri), the other buttons handle volume up/down or double-click for next/previous track. No need to lock/unlock the screen or even look at the screen. Just leave your phone in standby mode in your pocket.
It means I can use my iphone as an ipod without ever needing to take it out of my pocket. In fact I mostly use it to stream internet radio (Radio Paradise) rather than listen to pre-recorded audio, and the controls work just as well for that as for the ipod app. As well as listen to music while riding my motorbike, I can also have trapster running in the background keeping an eye out for, ahem, "safety cameras" (and conveniently muting the music to make announcements), I can get voice-guided GPS directions, and can hear when a call comes in and pull over to answer or make a call without even taking off my helmet, gloves, or taking the phone out of my pocket. I can even send text messages thanks to Siri while my phone is in my pocket. And I do! A number of times I've left home and realised I need to let somebody know I'm on my way, and I've managed to compose and send an SMS using Siri all while I'm stopped at a single red light. Of course on a motorbike the mic becomes useless while you're in motion thanks to wind-noise, so regardless of mobile phone laws it's not practical to use Siri on the move, but I've been surprised how much Siri still understands when I'm pulled over on the side of the road talking with cars whizzing past. Even with my motor still running.
The only problem I have is poor battery life. I ride a regular two hour trip every week which eats over 40% of the battery when streaming internet radio and running a GPS app like Trapster the whole time. If I just use the iPod app by itself instead it uses much less power.