In a Letterbox cache, you bring your own personal (hopefully home-made, not bought) ink stamper, and stamp the log book, and use the stamp in the letterbox to stamp your personal logbook.
Do you plan your geocaching well in advance, or do you like to hit caches on the fly, whenever you have free time?
If you're in the latter group, leave your caching supplies in the car – pens, trade items, gloves, boots – so that you can make a find at the last minute.