I use a headphone amp made by Vox called the 'amplug'.

The beauty of it is it plugs directly into my bass, and has an aux in which I run my iPhone into (you'll need a stereo mini cable to connect it - I use a Belkin retractable cable)

There are a number of different models for guitar depending on the sound you're after, and one bass model. Sound quality is pretty decent. Uses 2xAAA batteries which the website says will last about 15 hours (about right in my experience).

http://www.voxamps.com/amplug/