I constantly found myself in the following loop:
1) Motivated to study, study productively
2) Several days / weeks later productivity stops (for any number of reasons)
3) Quickly forget everything I learnt over the next month or so
4) Back at stage 1, feeling I have 'wasted' the last few months.
My big problem was the _forgetting_. Life is always going to get in the way, and I needed to 'drop anchor' when this happened, so I could resume where I left off, not start over.
I use Anki [1] to do this. I learn things, make flashcards, and spend dead time on public transport keeping up with them. As Anki uses spaced repetition, you can input a LOT of cards without this becoming overwhelming.
This gave me a sense of progress even when I did not study for a month, and massivly increased my motivation.