let's take for example pull-ups. for me it's one of the most difficult exercises to perform correctly and i was hardly able to make more than 14 reps per set. the reason why pullups are so tricky is that many people are physically disadvantaged for them: if you are tall and with long limbs (as i am), then pull-ups can be a tough game. still, greasing the groove can help you a lot.
how does it work? it's quite simple: all you have to do is to do on a daily basis how many rounds as possible, with a number of reps per round that corresponds to half of your max. for example, if your max is ten reps, then you have to do as many rounds as possible of five reps each through the whole day with a minimum rest period of 15 minutes between rounds.
obviously we won't be able to do everyday the same number of rounds, as we can't always have a pull-up bar at our disposal during all our waking hours, but that's part of the game: varying as much as possible the number of rounds performed daily is part of the technique. one day we will do 25 rounds, the next day 7, then 15 and 11 according to how much time we can spend near our pull-up bar. taking a day off once in a while, when our body requires it, is obviously allowed.
i won't go into details about how it works from a physiological point of view (let's just say that it helps our nervous system to learn the most effective way to use our muscles to perform a determined movement), i'll just add that in just three months i've been able to increase my pull-ups max reps from 14 to 19. if you're into bodyweight training, you'll know that this means a lot. it is recommended not to test your max more than once in a month to avoid overloading the nervous system and exerting the muscles. as usual in strength training, it's better to test the "massimale" after one or two days of rest.
still there? reach you pull-up bar and grease the groove!