How to deal best with systems or sidelines
Middlegame Planning
1. Expand your position:
a. Gain more space.
b. Improve the position of your pieces.
2. Decide on what side of the board to play.
a. Queenside: a-c files.
b. Center: d-e files.
c. Kingside: f-h files.
Compare, space, material, and weakness(es)
3. DO NOT HURRY. Regroup your pieces, and be patient.
"... there will come a time, whether on move two or move twenty, when your knowledge of theory runs out and you have to decide what to do on your own. ... sometimes you will leave theory first, sometimes your opponent. Nothing will stop this happening. It happens in every well-contested GM game at some point, usually a very significant point. This is a part of the game: an important part, ..." - IM John Cox (2006)
As one improves, one's games gradually tend to involve longer stretches that go according to book.
"... Review each of your games, identifying opening (and other) mistakes with the goal of not repeatedly making the same mistake. ... It is especially critical not to continually fall into opening traps – or even lines that result in difficult positions ..." - NM Dan Heisman (2007)
https://web.archive.org/web/20140627062646/http://www.chesscafe.com/text/heisman81.pdf