For languages, set up a deck with an SRS program like Anki (free), and try to learn 10 new words a day, and practice using one piece of grammar a day. It's only about 20 minutes of work but it's much less possible to cram for languages at A Level, so getting yourself into the habit of constantly using the language will really help you.
Also, do homework and do it well. There's less pressure at sixth form to follow all the rules, and teachers are generally less angry with you for not doing homework, but oftentimes homework will be something like making notes or doing a little bit of extra research, or doing past exam questions. All of these things are incredibly helpful at A Level, and a sustained work schedule throughout the year can really boost your summer grade.
So I guess if you want the tl;dr version: revise little, but revise often.