I've gone back and forth, paper, Excel, and tried a bunch of the online sites. I probably do best with Excel because it keeps me offline and I can set it up with only the fields I want. But I got lazy and didn't keep it up well.
I now use Training Peaks, which has crazy depth in terms of features but is simple enough for me to plug in my main activities and metrics and skip the rest. My favorite part is having a good weekly summary in terms of workout time per type (bike, crosstrain, etc.)
The main trick for me to use it, which I highly recommend for anyone using Mozilla Firefox as a browser, is to use the new "Pin as App Tab" feature (you have to upgrade to the most current version of Firefox). I love this -- every time I open Firefox there's the TP tab staring me in the face, asking me "what workout can you add now??" Weirdly enough it's more than a reminder, it's gotten me off the computer and out the door to my bike figuratively AND literally.
Sit bones = ~135 mm, saddles that work ~ 155cm/6.1 in wide
2003 da Vinci (custom road/all-rounder)/Terry Butterfly Ti
1994 Gary Fisher Nirvana (vintage MTB/commuter)/Terry Butterfly Chromoly
1991 Terry Symmetry (NOS frame/fork, project in progress)
1973 Raleigh Super Course (project in progress)