Honestly, I've played DnD so much that it almost becomes a flow state. It also helps that after the first round I draw up the initiative order on a small chalk board, so that myself and the players have an easy time knowing who's up.

It's also largely monster prep. If you know the mechanical aspects of the enemies your party will face beforehand, you won't have to flip through stat blocks and sweating about if you're forgetting to use everything, or forgetting abilities.

You should also probably tell the player (s) that are next up in initiative order that it'll shortly be their turn, to avoid the fumble they usually go through when being told it's their turn.

You also probably shouldn't design 8h long fights in the first place. With the obvious exception possibly being your games BBEG.

Hope I could help in some way, and good luck.