Others have mostly said the important things. Sadly, the 16MB flash has proven to be quite restrictive and as newer and newer features are added to RouterOS in every version its becoming increasingly tight.
General advice:
- avoid anything that may consume large amounts of storage: logs stored on disk, graphs stored on disk, large number of certificates, large address lists (thousands of addresses/prefixes, small ones are totally fine)
- basically the sure way to free up space is to netinstall the device
- a backup or export of your current configuration can be created even if no space is available on the device, but you manually have to specify the output file of the operation as being in the root of the filesystem (this is a ramdrive, so of course download it before rebooting)
- in my experience updating RouterOS leaves behind a varying amount of old configuration (which is logical - in case the naming, etc. of something was changed and you later choose to downgrade, it would be unfortunate to have the old config erased), and as a result of this I usually upgrade 16MB devices more rarely than others and do a full netinstall to the new version
- beta versions are quite often larger than the properly released ones - some hand optimization is probably done when promoting a beta to a stable version