The thing about terrorism is that its intent is to cause terror. Don't let it.
Grieve. Have sympathy. Be angry. Hope for competent professional police work. Demand fair justice for anyone who is caught and accused. Demand that lessons be learned, but that this not be used as an excuse for overreach. Grieve.
But don't give in to terror. If Boston area officials tell you to stay inside until they are pretty sure it is safe, and that there aren't more targets, listen to them. But don't give in to terror. Know that the TV media, in particular, will play this to increase their viewership, and that they will do so for weeks or months. But don't give in to terror.
Live your life. Cherish it. Cherish the lives of those around you. Don't give in to terror.
Grieve. Hug someone. Don't give in to terror.