Either I can't find subtitles for a great TV series or I need to read an article in a foreign newspaper.

If I can't understand what it says,  I get motivated to start reading again.

For me, before starting with a new language to learn, I stop and ask myself WHY. Persuading yourself with good reasons to keep up improving your skills in that specific language; for your career? post studies? communication? helping others? etc. I myself intend to learn some african languages in the future for preparing after graduation to do some volunteer work there, so as to speak with them easily and not talking to their minds only, but also their hearts. Accordingly, touching their problems. Multiply your reasons and find some extra motivations to wake you daily for learning more about it.  

Good question but its answer is pretty difficult. Being motivated is mostly a psychological issue. You should consider learning a language is such as your work or your study. You have a target in your life and have to reach it. Learning a new language in one step while working on your target. You should reward your self after each target you get. Additionally, keeping in touch with some people who is learning the same language will support you to be motivated and to keep working.

Cezar, I haven't tried to listen jokes on youtube. Maybe I will try it later. Thanks for those ideas :)

Mohammed: Yes, I've been trying to make a target and work on it. I do reward if I can achieve it. What about punishment? Do you give a punishment if you couldn't achieve the target?

Alaa: I totally agree that a reason is one of best way to keep the motivation. That's really nice to do some volunteer over there. What are you going to volunteer by the way?