"Play correspondence chess!" — Boris Spassky on being asked how to improve
"For sustained and lasting improvement, correspondence chess is unquestionably the most valuable method known." — Fred Reinfeld
"I often had as many as 150 games in progress. [It] had the useful effect of deepening my theoretical knowledge and giving me an insight into all aspects of the game. Later on this proved of great value to me in tournament play." — Paul Keres
"Correspondence chess and over-the-board chess complement each other." — Alexander Alekhine
Worth a read.
Although, beware. If you take up web-based or email correspondence chess, your ability to visualise in 3D might suffer - as it has for others . . .