Motivation is essential — at least to get started. I find that the works of Venerable Ledi Sayādaw are helpful.
(Also known as Saṃvega Gāthā).
Once concentration is well-established, and insight begins to arise, that will provide the driving force to continue. It is like starting a fire by rubbing two sticks together — it takes a lot of effort to get it going, but when it's well alight, all it needs is to add a bit more fuel now and then, and protect it from rain-water.
• • • • (Upasampadā: 24th June, 1979)