You get used to the first come first served for clubs thing.
I am EXTREMELY disorganised, and generally a bit shit. I do, however, tell my DC's to wave letters up my nose WHILE STILL IN THE PLAYGROUND. I keep a pen in my bag. They drag me to the office. It gets done.
I also now, after 8 years of DC's going through a school like this, have got used to the 'rhythm' of these letters - a new football (paid for) club letter out at the start of each term, new free clubs at the start of each half term etc.
The clubs are first come, first served, but with a caveat that each child can only do a term or half term in most of the free clubs. Only exception to this is for team sports.
I have learnt which times of year I need to pay for swimming, what year groups go which trips (and a rough cost, allowing for inflation and higher or lower coach costs depending on size of year group*).
*DD's year had 60, cost was X. DS1's year has 30, cost is 2X. DS2's year has 90, cost is 1.5X. Bit weird, but the coaches have 64 seats each. DD's year needed 1 coach, full up so each DC paid one 60th of the coach costs. DS1's year needs one coach, only half full up, so each DC pays one 30th of the coach cost. DS2's year needs 2 coaches, each having 45 DC's, so each DC pays one 45th of the coach cost.
But after a while, you 'know' this stuff, so you LOOK less disorganised. Even if you're not!