Memorizing anything comes down to one thing: repetition. Often, you will find yourself with a little bit of time to review something but you don't have your instrument. You can still get some valuable note name repetition in without touching your bass.
As I discussed in the how to practice articles, use the visualization technique. It really works. And, the more you do it, the better it starts to work for you.
The Paper Bass
If you don't like the visualization technique, here's something else to do. From the fretboard printer, print out a blank fretboard for your instrument. If you want to simulate the neck more, roll the paper up and tape it together and you have a mini-bass neck for practice! (Remember, you can adjust the width of the fretboard diagram to stretch out to the edges.)
Do the exercise: Tap your foot steadily like a metronome. Use your finger to touch the correct paper fret, say the note out loud, etc.
You can keep a paper bass anywhere: your desk at work, school locker, car, or even in the bathroom!
Practice your note names consistently and frequently anywhere you can. You won't learn them in one day, but learning them in a few weeks is possible. Practice!