Usually a fervent supporter of technology over anything, but I think I'd rather have that "huge calendar on the wall with a big red marker" than an iPhone (or web) app.
It's big. You wake up every day looking at it. And it brings us back in a connecting way to the real life.
Semi-unrelated: Same reason I'd still rather use a Moleskin to gather and write out my thoughts and ideas over TextMate, Photoshop, etc.
Streaks has been pretty effective for me; I used it to get myself to meditate every day. There's a little detail that makes all the difference: The app shows a badge on its icon that tells you the length of your streak. Skipped a day? No badge for you.
I put it on the home screen, so I get to see that badge many times a day, much more often than I would see a physical calendar.
It's big. You wake up every day looking at it. And it brings us back in a connecting way to the real life.
Semi-unrelated: Same reason I'd still rather use a Moleskin to gather and write out my thoughts and ideas over TextMate, Photoshop, etc.