Get a (writing) Notebook!
As a dilligent student, you should get a no-frills notebook/notepad (and pen) to keep with you all the time.
This notebook is incredibily useful, you can jot down your ideas anytime and anywhere. YOu can draw anything and write equations easily. You can also record down your research findings, experiment results, etc.
All the successful people I know carry with them a notebook.
- My supervisor at UT Austin, Prof Joydeep Ghosh
- The multi-millionair founders of Neonyoyo, Inc.
- The great Indian Mathematician, "Srinivasa Ramanujun":http://www.vidyapatha.com/scientists/ramanujun.php
Even failures like yours truly keep a notebook http://research.mosuma.com/Members/zhangguiyu/media/blog/notes_kuiyu/, so I really don't see how you can survive your career/studies without using a notebook.
Don't kid yourself by saying that you will keep notes using your mobile phone, PDA, computer, etc. It simply doesn't work. I have tried doing that before, it does not work. By the time you get your machine out, your ideas already gone. Also, try writing an equation, and drawing something, it takes you ages.
A notebook is the simplest apparatus, it tracks your ideas sequentially. Which means you can also look them up sequentially.
Nothing beats the good old pen and paper.
I hope you will seriously consider my suggestion.

