TLDR
- Move as many things as possible in regular life to the target language (software, tv shows, reading, etc) to maintain the language
- There is no substitute for full immersion if you have the opportunity. I learned traditional Chinese in Taiwan while I was an ESL teacher.
- The power of SRS (spaced repetition system) cannot be overstated
I will add my own bit to it. If you like mnemonic systems, Heisig wrote a very good book called "Remembering Traditional Hanzi" where the idea is to use the radicals in the character to construct a visual image to aid in recall. I highly recommend it.
https://uhpress.hawaii.edu/title/remembering-traditional-han...