Rubato is also the excuse lots of pianists use to slow down for passages they can't do properly, or to excuse a poor sense of timing/rhythm.To improve your use of rubato, you need to improve your ability to feel the music and improve your underlying sense of the pulse of a piece.I'm also going to stick my neck out and disagree that rubato has no place in music before the Romantic. The metronome was invented in 1815, and rubato appears to have been invented about a nano-second later! I'm guessing a certain flexibility was the norm before then, though how much could be debated (I'd suggest restraint).
The way I was taught, rubato is referring to rhythmic and expressive freedom without having to strictly adhere to the stated tempo. But also remember not to exaggerate too much otherwise it will create a very unmusical sound, something that resembles Lang Lang a lot. JL