Feeding Pet: When you feed a pet and are done using/playing, select a different pet. And that timer will stop and the pet will stay active.
Get more rare cards: Some people will give away free rare cards on Facebook groups. Ones you get one you will start getting more of the same easier when opening new chests. The extras you get you can trade for new rare cards.
Super betting: If you get super bet enabled and never go under 5 spins. You will keep super betting at x5 as long as you don't go under.
To complete more sets and win more spins my best recommendation is to buy chests for at least 1 billion on each level to village 50 after that you should buy for at least 1.5-2 billion to make sure to get most cards that can be found on each level. Also, try to save to 7-10k spins so you can bet x50 when spinning.
When spinning, try to learn the patterns and don't bet x50 every time. Also when you feel that you are about to lose more spins than you win it's time to stop and wait for the next event.
Only buy chests on cards boom event. At least that is what I do and it's working out for me. You can check out my card finds from level 1 to 55 here: https://coinmastervn.com/my-card-finds
It's also good to have a lot of friends to raid. The more friends you have, the more coins you will win.
Do this and you will become a "Coin Master". This strategy has worked for me on my second account where I have 31k spins and most sets completed on village 55.