The biggest advantage that amateurs can have over professionals is behaving randomly. If amateurs do something randomly or without any reason, it can be difficult or impossible for professionals(and other opponents) to deduce what range of hands he hold. For example, if the amateur raises because he feels like it, it will be very difficult for a professional player to realize the reason behind his actions and behave accordingly.
Unfortunately for most amateurs, they do not use this strategy and amount to “doing what they feel” which makes it relatively easy for a professional to figure out why the amateur takes the actions he takes. For example if an amateur checks a strong hand on the river when a flush card comes in, it is generally because he is worried about being beaten. Thus, in the future, when a flush card comes in, a professional will deduce that it is much more likely that the amateur has a flush when he bets. If the amateur threw a wrench into the professionals hand reading, for example, bluffing when the flush possibility was in spades, the professional would almost never figure it out and would be folding more often than he should.
This idea became clear to me when watching a hand played by an excellent live player who I will call Don. The game was $5/10 no limit with $4000 effective stacks. He raised preflop to $40 with 6d5d and was reraised by a bad amateur player to $150 which Don elected to call, knowing that the amateur almost surely had a big hand, either QQ, KK or AA.
The flop came Ts6c2d and the amateur bet $300(almost the size of the pot) after Don checked. This almost certainly meant that the amateur had a big pair and Don knew it. Don elected to checkraise to $800 hoping that the amateur wouldn’t want to put in so much money with his big pair, in addition to having a pair and a backdoor straight and flush draw in case he amateur called him. The amateur elects to call.
The turn is a 7d, and Don elects to bet $1900. Sitting at the table, I knew that Don just had one pair and was hoping the amateur would fold to this big bet. The reason I knew this was because Don was a good player and almost certainly would have bet less if he wanted to be called, because that would make it more likely he would be, then make it more likely he would be called on the river. For example, a much better bet sizing on the turn with about $3000 left would be $1200 on the turn and $1800 on the river and I knew that Don knew it. So why would he bet so much unless he was trying to induce more folds from the amateur? As it turned out, the amateur thought a long time, and eventually went allin with QQ and Don called, missed his flush draw and his two pair draw and lost. I realized after that Don actually should have shoved the turn to induce the maximum amount of fold equity from the amateur.
Knowing that the Don would make the optimal play against an unknown opponent allowed me (and possibly his opponent) to decide that he did not have QQ beat and went allin instead of folding. In these situations professionals can be somewhat restrained by knowing what the optimal way to play the hand is and not wanting to veer too far away from that. This leaves most of them playing relatively formulaic and makes it easier for good opponents to figure out what they are holding. Adding a bit of randomness or doing things a bit oddly can make it much more difficult for your opponents to read your hand and can entirely change your image at the table. So, randomly open to $100 on the button instead of $35 the first time your are on the button and the new dealer just sat and include whatever other randomness you can in your game.