Well, it's not killer per se. If you've had a bit of experience you probably already do this. I've just started to document what I'm doing with each of my projects so I can start right off the next time I want to work on it (instead of spending a half hour figuring out what the next step is). I just create a file called TODO.txt in the main directory for each of my projects and I update it every time I work on the project saying what I did and what's probably next. Hopefully that will keep me up to date on what I've done.
Now here's the REAL killer tip: automatic timestamp. If you use Notepad++ hit Ctrl+F5 to automatically enter the date/time where the cursor is. And this, THIS is the real killer. It comes from good old Notepad. That's right, Notepad.exe takes the cake for awesome features with this beauty: create a new TXT document and type '.LOG' (no quotes) on the first line. Save it, close it, then reopen it. The date and time is automatically entered right above your cursor. Just start typing.
I can't believe Notepad of all things has such a useful feature. It doesn't even have proper 80 character line wrapping.
(Edit) I've done a little searching and found out that you can also add the timestamp in Notepad by hitting F5. Not nearly as cool though.