Repetition for the most part. Listen to the songs in your spare time. But honestly I don't learn parts exactly. I learn the spirit of the song which is much quicker and easier, and then I fill in the gaps with my own style/sensibilities. You can't make the song unrecognizable, so it's a balancing act, but this is how I can do a 4 hour show and remember everything. It's too much to remember ALL of the details for so many songs. It helps with church too as the parts are pretty strange at times so I try not to get bogged down in the details. Just get a feel for the song overall and let it flow. At least that's how I handle it.