The beginning of a new semester is the time to get organized. While I still do make use of the ever-present yellow Post-It Notes of various sizes (the physical variety), I have found that one big disadvantage of them is that they are only in one place. More and more all of us are using a tablet, a smart phone, and... oh yeah... the PC to keep track of what we need to do. One GREAT cross-platform FREE software is Evernote. I recommend giving it a whirl. I have upgraded to the Premium version, but could probably get along just fine with the free version.
Here are some things that you can do with it.
Find a Tweet, or get an email that you would like to save? Are you on your phone and not at your desk? Your Evernote account comes with an email address, so you can simply forward whatever you have to Evernote. I have a neat app on my iPhone called CamScanner. It turns a camera shot into a PDF which I can then email. One way to use this for business is to scan those restaurant receipts, turn them into PDF's, and email to Evernote. You can then organize them according to the consulting project in Evernote.
Another nice feature of Evernote is that the notes that you save (which can contain pictures, text, audio, or video) can be organized into Notebooks. Notebooks can be organized into Stacks. You might have a "Stack" for Class Notes. Under that Stack, you have a Notebook for each class that you are currently taking. Notes can have Tags, which can be searched for. For example, use the Tag "Assignment" for class assignments.
For an extensive list of the ways to use Evernote, try THIS article from Lifehacker.com.