I think it's well accepted that doing exercises is not only the most effective, but maybe the only effective approach when learning quantitative skills.
But what do you do with the tons of exercises you've done? Is it worth putting them in $\LaTeX$ and saving for posterity? Does it make any difference? If I've done most of the exercises of a given textbook, I've obviously trained myself, so is it ok to now throw the solutions in the thrash?