For me, and for a lot of other people, a smartphone is something of a blessing and a curse. What's wrong with smartphones, for me, is easily summed up: they distract.
This is most obvious when your phone actually chimes at you in the middle of something else you're doing. Talking with friends at a restaurant when, out of the blue, "ding!" I can't abide having this happen to me, so I've turned off audio alerts for emails and even most text messages. (I do allow alerts for calendar entries, even mid-meeting at work.) This is "Don't bug me!" distraction avoidance.
There's a more subtle way a smartphone distracts, though: when you're trying to use the phone for one purpose, then get "derailed" by something else. This is not unique to phones; computers are common culprits as well. That said, it's more egregious on a phone simply because of how often you are "in and out" with a phone. Most "transactions" with a phone are short and have a certain purpose, so when you turn on the phone to, say, record a thought, only to be presented with the latest news on Afghanistan, it is frustrating to say the least. Dodging this requires "Get out of my way!" planning.
I have tried to work around this by having email not put up any notification at all when new mail arrives. Instead, I have to go looking in the mail app itself for any updates. This means that my turnaround time on mail is a bit slower, but I lose less of my thought process in a quick phone "transaction".
The other mitigation I have attempted is making it possible to do some operations with the screen off. Voice recording is often the most convenient for me to get something off my mind, and I have set up my phone so that tapping the volume down button with the screen off immediately starts recording a note for later review. It has been really effective and lets me note something down without getting "sucked in" by whatever notifications my phone has to offer.
Instant-on camera is another feature that lets me use the phone without getting sucked in. Just holding the volume button turns the phone on, right to the camera. In and out!
Are there strategies you use to keep your focus in your control?