It is surprising that the Classical Sicilian isn't more popular, because if White wants to play for an advantage his options are rather limited, while Black has a range of reasonable systems. John Nunn, Grandmaster Chess Move By Move.
It is usually easier to win with a substantial positional advantage than with an equally substantial material advantage. Andrew Soltis, What It Takes To Become A Chess Master
A lead in development is an ephemeral advantage; it cannot grow indefinitely. The proper technique to exploit a superior mobilisation of forces is to transform it into a more lasting sort of advantage. Lluís Comas Fabregó, True Lies In Chess
After every game, try to think of three new things you have learned. You will find this is not always so easy, but it is a very useful exercise nonetheless. Jacob Aagaard, Excelling At Chess
In planning your game, arrange your pieces in such a way that they are of maximum use and cooperate well with one another. Strive for harmony! Vladimir Popov, Chess Lessons