sly student | intermediate-advanced
Their Mistakes
This is a technique which can be done very randomly on any occasion when the possibility presents itself, but if desired it could also be done systematically by searching for videos of native speakers of the language you desire who are speaking your mother tongue as a second / learned language.
The idea is to observe the mistakes they make while speaking your mother tongue and to trace the roots of these mistakes and identify which grammatical phenomenon it is linked to. Hearing such mistakes in your mother-tongue can often greatly assist in understanding the grammar of your desired language.
For people who really thrive with this exercise, you can keep a mistake diary in which you notate these mistakes and their origins.