I used to print out spell sheets and put them into some sort of a spell booklet for reasons like this. It also helps that I played a bard when I did that and so there wasn't a whole lot of spells that I needed to keep track of.
The last time I played a wizard, I just relied on a similar tactic (one-spell sheets compiled into some sort of a booklet), but instead of printing it out, they just stayed as files that I move to a directory on my phone called "prepared spells".