This are just some aspects that have been working for me as an adult learner, I wish they may be somewhat helpful to you.
Define the use of the information
Before staring any learning project I always assess what I need to do with the information. Do I simply just need to know it, or just be able to combine it with some other pieces of information, or be able to criticize it, or even be able to create something new from it. It's important to know what I called your "learning verb" before learning cause that can affect your approach considerably. To learn more, read up on Bloom's Taxonomy.
Cultivate a higher meaning
In college we study for grades. In adult learning there is no grade. I believe it's crucial to have some form of advanced reasons for your learning. It can be to build up your legacy, to feel a certain state of mental excitement, to prepare for a professional exam, etc. For me, I usually base my meaning on being able educate others.
Learn multiple subjects simultaneously
This may not work for someone but I need to learn multiple things the same time. For i) it's less likely to bet bored and ii) sometimes different subjects can form amazing synergy or interesting contrast. For instance, if you learn programming and Spanish, you can write a program that process Spanish texts, or you can try translating some programming text into Spanish.
Build up your repertoire
Be ready to show what you have learned and what you can do. When you told your employer that you have learned something, and he/she asks what you have done, you CANNOT just hand in a copy of your self-tests based on the books' questions at the end of each chapter. Think up creative ways that you can showcase your knowledge: professional qualifications, blogs, relevant work experiences, small but nicely functional projects, pieces you have created, etc.
Record your progress
A lot of us may overlook documenting our learning progress but it's actually amazingly helpful to me. A day is long, but a year is actually quite short. We can have done and achieved a lot without even noticing any progress. A small reminder for each little achievement in learning can become a powerful motivation (and a repertoire itself) when you feel running out of gas.
Incorporate what you learn into your life
I think for myself the most important point is to apply what I learn into everyday life. I read other people's blogs, try to answer questions of others, join and form interest groups, perform in front of other people etc. Once the materials you have established a closer connection to you life, you're less likely to feel bored and quit.