Signifying something as a 'dirty trick' presupposes mutually-agreed upon standards of conduct in warfare that it would contravene, and for most of (European) history this has not really been the case. General principles (capture rather than kill nobility, for instance) may have been perceptible but usually those had an underlying layer of practicality (prisoners/hostages are more politically useful than corpses). You can find many examples throughout history of military subterfuge, but not much stuff that was roundly condemned as 'dirty.' Even those were usually isolated or unique events. If some 'secret trick' allowed a smaller army to regularly rout a larger one, regardless of circumstance... Would it really stay secret for long?
As a side note, GoT's fighting and warfare owes much, much more to Errol Flynn and lord of the rings than to history. Don't put too much stock in it.
Deception plays an important role in all martial systems, from movements of armies to the movements of a body in a duel; what you call dirty tricks, martial writings call sound tactics. In dueling, the most basic and common deception is the feint: you attack a target knowing your opponent will react, and you redirect your attack to counter that reaction.
But you want something more fun... In his 1606 fencing manual, Salvatore Fabris takes a brief look at the rapier with a cloak as an off hand weapon. He shows an action where the fencer throws his cloak at his opponent's face, and as it opens up and moves forward through the air he puts the point of his sword into the cloak and thrusts forward, through the cape, into the face of his opponent! You can find the (almost) original illustration of this with a quick google image search.
You might want to head over to /r/wma and ask in there, there's certainly plenty of dirty tricks as well as some straight up nasty ones in the old manuals. In the meantime, you may find this amusing:
Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, questions of this type can be directed to more appropriate subreddits, such as /r/history or /r/askhistory.
How about this:
During the seige of Caffa (in what is now Ukraine) the Mongols hurled the bodies of those that died from plague over the walls using catapults.
That was in 1346, and may have been a major cause of the spread of the Black Plague into the rest of Europe.
If I can remember correctly from P. Contamine's War in the Middle Ages shields were generally made of wood, which means that there likely wouldn't have been much of a reflective surface to reflect light off of.
Another "trick" used in A Game of Thrones is when one duelist wears out another due to the weight of his armor. GRRM should know better, but medieval armor wasn't particularly heavy; its total weight is less than a modern infantry pack, and the weight is distributed pretty well across the shoulders and waist. A fully-armored knight or man-at-arms that had fallen from his horse would've been back on their feet fairly instantly. A trained soldier in a suit of armor would be unlikely to tire any faster than anyone else, so depending on them to wouldn't be a trick you'd want to stake your life on.
A lot of medieval dueling is a bit more like brawling than the more fluid image of duel one might have. "Get close" seems to often be step 1; when you're close you can slice, or half-sword a blade into the gaps in the armor. You can also knock your opponent down, jump on their chest and stab into the gaps with either sword or dagger. Yes, I'm making this sound easier than it is.
You can see most of the things I've talked about in this video from the Musée National du Moyen-Âge de Cluny