This will be a brief lesson, being as every situation is different than the last.

I was just casually playing incognito earlier today, when I noticed how foolish some people are. This replay should easily demonstrate my meaning. Now tell me, what should he have done here? Well, that's not so important as what he did wrong, why he did wrong, and when he did wrong. Notice in a deck like Thunder-Family, that there's a lot of hand reveal. He knew I added two Prisms to my hand, but he let me get out my Synchro play regardless. It's a simple concept, really. He was expecting a Stardust Dragon, knowing I'd attack head on, ignoring his backrow, so he'd use Fiendish Chain and stop it... But he instead got Manga Stardust Dragon which started a chain that led ultimately to an OTK. If this guy was playing smarter, he would have limited me to the 2 monster Rank 4 XYZ by using Fiendish Chain on the seemingly meager Mahunder, negating the Normal Summon, and limiting me to either the viable Thunder King, or a Rank 4. Now why is this relevant? Because you should always try and think ahead. Letting me get out a Synchro, he's not only limiting his own special summoning by allowing Rai-Oh free reign, but he's also letting me chain manga Stardust to it. Usually, this isn't a problem. When you have Reborn in hand, though... I would have done 8200 damage, rather than at most 2600 if he had chosen Mahunder instead.

In short, the simple message is to always evaluate the situation. Try to put yourself in their place, and think "What would I do?". It doesn't always mean you'll win, but it's worth mentioning. If you know something in the opponent's hand, it might be worth considering the option of stopping them from using it all together, rather than trying to be coy and negating an unknown level 8 synchro. Use that Mind Crush you sided on something that can set up for disaster later, like a Gagaga Girl! If your opponent throws down a Tour Guide, considering using Effect Veiler on said Tour Guide, rather than what they're going to summon with it. Torrential those Six Samurai just before Shi En is summoned. It's the little things in this game that make the biggest difference. Food for thought.

I've seen that happen way too often against my frog deck. People aren't used to Des Frog, so they bottomless trap hole the first one rather than waiting to get both of the other ones with it instead.