I was in Istanbul when it happened, and living in Galata - so pretty close. In fact, I happened to be walking along Istiklal Caddesi hours before the bombing took place.

Police is definitely more on the alert, the main thoroughfares - think Eminonu, or Besiktas - have multiple patrols with armed police. The ferry terminal for Kadikoy, for instance, was always guarded by officers with armored cars, not the usual vans/sedans.

Another thing I've noticed is that metal detectors have appeared at the entrance of the metro stations and some ferry terminals (Besiktas is one of them). They do a pretty quick check of bags, but it's something I'd never seen in my 10 visits before.