For keeping track in session (and even for between sessions) I used a token system with my players. I ran out to the dollar store and bought a bag of colored beads. Distribute to your players as needed. Red beads are how many rages you have left. Each color represents different level spell slots, etc. When you spend a slot, you give the bead to the DM. When you rest, your DM gives you the beads back. When you go home at the end of the session, you take your remaining beads with you along with your character sheet (throw them in a ziplock bag or keep them in your dice bag, whatever). The DM takes all the beads that players gave him home with them. (they can be stored in bulk/mixed between all players - since the only thing relevant is color, when players rest they just ask for x amount of y color beads).
Players can manage the beads without the DM getting involved, too - just have a bowl in the middle of the table that players toss their expended beads in. When they rest, they reach in to the bowl and pull out however many of each bead they need to reclaim. DM doesn't have to get involved at all other than to take them home at the end of the session.
The advantage here is that you don't have to worry about your paper wearing down. If you use a card or an alternative character sheet, you end up constantly erasing and re-writing your number as you spend and regain slots. Eventually, you'll end up wearing a hole straight through the paper; not to mention that things usually don't erase perfectly, leaving shadows of numbers previously written down, overlapping over and over, making it harder to read.
The beads are cheap and come in bulk, so no harm done if some get lost or w/e. I personally try to get the glass beads that are disk-shaped rather than spheres so that they can lay on the table without rolling away; but you can get more color variety (and larger bulk) if you buy plastic beads, and especially if you get the round ones.
Now, that being said, when it comes to longer extended breaks, I have to ask...
Does it matter?
Within a session, or when jumping between two sessions, yes it typically is a good idea to keep track of how many spells or rages or w/e you have left. But if you just took an extended break, does it really matter when you come back? I would posit that most of the time, it doesn't. If you have all of your rages available or only one left wont make a big impact unless you left off just before the final boss of a dungeon or something. If the DM knows that there's only one such encounter left, then it'd probably be best to just do said encounter before wrapping up for the night.
As a DM, I would typically just handwave it and say "lets just assume you all took a rest and are back to full". The vast majority of the time, it wont make a difference.