Two different problems, two different kinds of aid
Most memory complaints are actually one of two different problems, and they need different tools. Prospective memory is remembering to do something at the right future moment — take medication at 8am, call someone back, leave for an appointment on time. Retention is holding onto information you've learned so you can recall it later — a name, a fact, a procedure. Calendars, timers, and consistent object placement address the first problem. Spaced repetition and mnemonics address the second. Using a retention tool to solve a prospective-memory problem (or vice versa) is a common reason people give up on a technique that would otherwise have worked.
Calendars and reminder systems
Best for: recurring or future-dated tasks (bills, appointments, medication schedules). The mechanism is external and reliable — the reminder fires whether or not you happen to think about the task, which is the entire point. The most common failure mode is having reminders in more than one place (a paper calendar and a phone app and sticky notes) so that none of them is trusted as "the" source; picking one system and retiring the others matters more than which system you pick.
Timers for focus blocks
Best for: staying with a task long enough to finish it, and noticing when attention has drifted. A timer doesn't improve memory directly — it structures attention so that whatever memory-dependent work you're doing (reading, studying, a multi-step chore) gets done in a state where you're actually paying attention, rather than technically present but mentally elsewhere. Short blocks with a real break in between tend to outlast long unstructured stretches.
Mnemonics
Best for: retaining specific, bounded pieces of information — a short list, a sequence, a name paired with a fact. Mnemonics work by attaching new information to something you already have solid access to (a familiar image, a rhyme, a story), which gives your brain more than one path back to the information. They are less useful for open-ended understanding of a topic, and effort spent building an elaborate mnemonic for something you'll only need once is rarely worth it.
Paper planners
Best for: people who retain information better through handwriting than typing, and for tasks that benefit from seeing a whole week at a glance rather than one notification at a time. The physical act of writing something down is itself a memory aid independent of whether you ever look at the page again — this is different from a digital calendar, where the reminder does the work and the writing itself doesn't need to be remembered.
Spaced repetition
Best for: retaining information over the long term, especially material you need to keep recalling for weeks or months (vocabulary, exam material, names of people you meet regularly). The technique deliberately reviews information right around when you're about to forget it, rather than reviewing it either too soon (wastes time) or too late (you've already lost it and have to relearn from scratch). It requires more upfront setup than the other tools here — usually some kind of scheduled review list — which is why it is worth using specifically for information you actually need to keep long-term, not for one-off tasks.
Light and environment
Best for: the more indirect problem of attention and mood affecting how well any of the tools above actually get used. Poor lighting, especially during darker months or in windowless workspaces, is associated with lower alertness and motivation, which can make even a well-designed reminder system harder to act on consistently. This one is not a memory technique by itself — it's an environmental condition that makes the other techniques more or less likely to actually happen.
External memory aids generally
The common thread across calendars, timers, paper planners, and even light is that they move the "remembering" work outside your head and into your environment. That is a legitimate, well-supported strategy, not a shortcut or a sign of a "worse" memory — offloading routine remembering to a reliable system frees attention for the tasks that actually require thinking.