What laragtija says is very good.
Also, listen to recordings by good players - hear how they approach rubato. You won't feel that everything you hear will sound "right" to you - but think of what, to you, works and what doesn't.
Listen to other instruments e.g. piano, violin, cello, etc, etc.
Rubato is a funny old thing - it can easily be over done or employed incorrectly, but when it is done well it just feels "right" and lets the music breath. Also, the subtle use of rubato can be very effective.
Forgive me for referring to the piano but as an illustration in restraint and rubato I always find this slow movement from Schubert's D960 piano sonata fascinating (it is also one of the most beautiful movements for the piano that I know).
Performances by three very great musicians.
There is one version where rubato is used a lot to increase the expression (perhaps needlessly).
Another where it is used less but "micro-rubato" infuses the playing and gives it tension and "life"
Yet another where rubato is used more sparingly and this performance of the movement always makes me cry.
You can just sample the first couple of minutes or so (but the whole movement is wonderful). I'll just give the links - [Edit] now in the same order as above