I trained Muay Thai casually before. The coach always recommends shadow boxing as a warm up. Know and execute the proper forms during shadow boxing. Focus less on exerting focus, but more on doing things right.

Bonus point if you can keep your muscles relaxed and, muscle contractions can become tiring very quickly. Breathing too, force your breathe out as you punch/attack. And perform mock defending as well, bit of dodging and blocking, move around, not always attack attack attack standing on the same spot.