People say: "Write a little every day!" The good thing? No one says you must continue writing your WIP. Nathan Bransford recently posted on how he can't write every single day. He needs distractions of some kind. And if you're looking to vent, need some time to sort out a particularly tumultuous day or want a bit of serenity and reflection, diaries are the perfect solution.
I think we need to address the problem of "diaries are for girls" right now, though.
First of all, there's the general aversion to the word "diary". FYI, "Dear Journal" works just as well. So does "Yo, ma tough homie notebook" (you know, just in case a thug decided to crack open a Moleskine). A discreetly coloured notebook will also let you answer the question "Dude, why're you carrying around that book?" with "This is confidential material unsuited for the regular civilian's eyes." Learning how to waggle your eyebrows like a CSI agent on TV would be beneficial as well. However, this obviously doesn't work with pink notebooks. Just saying.
Next, the question: what to write? Here's where anything goes. The purpose of a diary/journal/tough homie notebook is to get words down on paper. Trains and trains of thoughts can chug in here to get a maintenance check and let off some steam. If recapping the day's events isn't interesting at all to you, give yourself permission to let loose all those snarling thoughts about your boss's annoying tea addiction, or how you hate the weird dog that's always sitting in a puddle of its own pee at the corner of your block.
If all else fails: write song lyrics. I think this method works tremendously because everyone has a song or two that they relate to, that they consider their song. Record how the lyrics make you feel, how the crescendo over the bridge can always make you catch your breath, and the way the final beat of the bass goes right through your chest.
No matter what you write, your diary will be filled with genuine emotion. It's personal, it's real; and being able to transfer that emotion into your writing is powerful. Another thing people say often is "Write what you know". And what do you know better than your own life story?