Most experienced will tell you any small thing you do or avoid can cost you the match or win you the match. The majority of the tricks you can do to help win without cheating are in real life situations instead of the emulated games on DN or YGOPRO. I will be talking about several tricks that can get you just a bit more of an edge in the duel.
1. Do not pass up the opportunity to look at your opponent's deck or hand!
There are several reasons you can get a free peek at your opponent's deck. When your opponent triggers a required search can not use it as there are no legal targets, or he/she uses a card that has a cost that makes the search not legal (such as charge of the light brigade), then you have a right to look at his/her deck and it is highly recommended you do so you can deduce what he/she has in his/her hand, as well as what to expect coming up. You can often look at your opponent's hand with effects such as mind crush. Even if they discard, you have a right to look at their hand to check for any other copies. Remember that any time a card is revealed to you, you have a right to check the card name, effect, and everything else about the card.
2. Know what cards you don't want to see played against you.
There are many opportunities for people to side in common side deck cards to counter your deck. Knowing what common cards your opponent may side against you can help you develop a side deck and use cards to counter them second or third game, preferably ones that can be used in other situations like Mystical Space Typhoon, effect veiler, dark bribe, or breakthrough skill.
3. Know the Meta
Knowing the meta can easily help with what you should place into the side deck as well as the main deck. Searches has become so common that using a card like mistake in the main deck is almost never a hindrance.
I tried to keep these tricks specific to online situations. If you have any situation online or irl life that was not mentioned then you can explain it here for 40 Crystal Shards.