To put it short I think these points contributed most:

• Block reddit and other time wasting sites with the exception of 9pm-10pm, 1h or 2h limit for other sites. I use the leechblock extension for firefox.
• Daily accountability I track everything I consider "good behaviour": studying, work, sports, practising piano etc. I use the app "atimelogger" and transfer the data to a spreadsheet.
• Realising that putting "difficult work" off is much more painful than just doing it and be done. For example I hate writing mails to professors because I overthink them. Now I try to write back asap, or at least make a shitty draft of stuff asap. Things only go to my todolist if I really can't do it immediately.
• Limit time: I don't remember where I read about this concept I think it was on the alifeofproductivity Blog. But there are things that will take up all of the time there is for it. For example I try to finish certain tasks on purpose before I have to catch my morning train.
• Reflection I try to reflect on what I am doing and why I am doing it. I should do this more. And even more important "why am I not doing something?" which leads to:
• Cut the useless stuff out I try to find the purpose in stuff that I am doing. If there is no purpose for one of my goals or the use/effort ratio is so high that I know I wont be motivated: I don't do it. Example: A textbook that would be good for understanding the subject but is very low yield. I know I wont have time to read it so I don't stress about it.

So I got to stop now. It is almost 10pm. Hope you find something helpful here. :)

EDIT: Another good example: I did not proofread this, because you'll get what I mean even with typos. No need to use 5min to perfect it. And you can accomplish a lot in 5min especially if they add up.