I just have a series of "pips" that I colour-in when used and erase when claimed back. Super simple, easy to see at-a-glance, and robust so it's not going to get messed up in my bag. Added bonus is that it works when being DM too and you have several casters to track simultaneously.
I have seen people that don't track spell slots for NPCs and just have them all at-will, which I think is an interesting idea. But I tend to give players non-combat objectives in their encounters, which prolongs them significantly so spell slot usage can become important for balance for NPCs in those cases.