Some tips, in no particular order:
1. A good way to use MST and similar cards is to set them face-down and wait for your opponent to set a card of their own, then immediately destroying it with your set MST during their End Phase.
2. If you control no monsters and have MST in your hand, it is almost always a good idea to destroy 1 of your opponent's set cards with it
before you summon anything. Sometime you'll hit a dud, but most times you'll dismantle something that would screw your newly summoned monster.
3. You should pay attention to whether your deck is strong or weak when top-decking, i.e. relying solely on the card you draw during a turn. If it is, you should do all you can to stall an opponent whose deck is weak when top-decking. If not, you have to be aggresive and end the duel before you hit that stage of the game.
4. If at the end of your turn you have exactly 2 monsters that you can Xyz/Synchro summon with, don't be tempted to always do so. While a single big monster is stronger than 2 smaller ones, it is also a sitting duck for most removal cards and monster-stealing cards like Creature Swap. Hence, sometimes it is actually better to leave those 2 monsters the way they are, depending on what you have in your backrow.
5. Some observetions on the number of copies of a card in a deck:
If you only have 1 copy, unless you have a couple of good ways to search it, expect it to only pop up in every second duel.
If you have 2, you can expect 1 copy in nigh every duel.
If you have 3, you are pretty much guaranteed at least 1 copy, and will get 2 in every third duel.