1. Get a good 18650 battery by a dealer who knows what he's speaking about (and who has done some test to be able to tell it's not fake batteries).
   Maybe I was just unlucky, but the red UltraFire battery I got from eBay was just crap. The good one was a Sanyo/EVVA 3400 Protégé got from PuissanceLed:  
    
   

2. Get this 1€ battery charging board, and use this wiring:
    
   

3. Ready to run for more than 7 hours!

During this test, the SamplerBox software was constantly running (so there was a non-negligible CPU usage), and these devices were connected to the RaspberryPi, taking some power:

• a USB DAC (for better audio quality),
• a USB MIDI keyboard (Akai LPK25),
• a USB SD card reader,
• a 7-segment 4-digits display,

so this means it could even be longer without these devices! I also read somewhere that it's possible to completely disable USB hubs, this could be useful for some other project.

And some other ideas for even longer battery-life (see this discussion):

• CPU throttling (allowing to decrease from 700Mhz to as low as 100Mhz when the RaspberryPi doesn't need much power),
• disable Ethernet port,
• plus many other ideas in this discussion...