I usually follow the Dungeons and Dragons 5e rules pretty well but there are three rules I like to use in my campaigns that are not in the standard rules:

1. Second natural 20 instant kill – I think this was a rule in prior version or I imagined it. But when a player rolls a natural 20, I get them to re-roll a D20 and if it is a natural 20 again it is an instant kill. Doesn’t matter if it is a Stirg or Black Dragon. It is dead. It is amazing how much fun and excitement this rule brings.
2. Written recaps – This was suggested by “How to be a great GM”. I write a 2-3 page session recap after every session and send it out to the players. This helps to capture what was done and it helps me hone my storytelling skills as well. The unexpected benefit is at the end of the campaign we have a 40-50 page keepsake of the story of the campaign! This is a bit of work but the players all look forwards to it.
3. Adding detail to campaigns – I love to take existing campaigns and add more needed detail. Unique NPC names, festivals, stores, puzzles, and events. I take the canon campaigns and add detail that all players look forward to. They get to know there will be unique stores, markets, events, and puzzles they can visit and solve. Sometimes we have a session of them just exploring towns. Of course, this is balanced next time with battles, combat, and adventuring. But the addition of details, names, motivations, and character depth makes towns and NPCs so much more fun.