How to improve rating on all chess variants

crazyhouse 1|0- build a box around your king while winning on time. pawns are best for this situation because they have the best ability to defend.

Crazyhouse 3|0 and up- The strategy is to work at your regular chess as well. This is necessary in order to improve your game. Another strategy is to save your pieces for the perfect moment. trading pieces also is a good idea in order to have more options for checkmate.

King Of The Hill- You cant be to aggressive in the beginning, this causes lack of development causing the opponents king to take control of the center four squares. Instead be patient and develop material. This is the only game mode where castling doesn't help you always. Controlling the center is everything.

3-check- another game that you shouldn't be aggressive in. sacrificing pieces including you queen is worth it with lower ratings however setting up pieces for 3 checks is also good. Not as hard as the other variants.

chess 960- I don't play this variant however I do know that it is vital to get good at regular chess to improve in chess 960.

So anyway there are the big ones. hope this helps.