To get the most battery life from the Clip+, use mp3 files. Other file types, especially protected wma files will eat battery power much faster. Keep the screen brightness at a minimum(as low as you can while still being able to see the screen). Set the screen time to a low number. Set the equilizer in the neutral(off position). Let the player play albums straight through, rather than choosing individual songs. Press buttons as little as possible.
If you still need more battery life, then get a usb battery pack. The $30 Veho Pebble XT is compact, and will probably give around 10 full charges to the Clip+. The Clip+ will play while it is charging if you use a usb battery pack, an AC usb adapter(even a mini usb cell phone charger), or via a pc if you use a power only cable. USB battery packs usually come with a power only usb cable. Another good usb battery pack is the $15 Tekkeon mp1580. The mp1580 uses 4 nimh AA batteries(not included), which makes it larger and heavier than the Pebble XT.It also holds less power, however one can carry as many extra sets of charged nimh batteries they want. The Pebble XT has a built in Lion battery.