What are some things that can be done when you start having "those" feelings again?

Cutting to the chase: I have issues with social anxieties and clinical depression. I've battled them for years and years. Sometimes weeks or months can go by without a single negative thought. Then, something rather insignificant happens and I can almost feel myself sliding back down my mental health ladder all over again. This morning, I'm having those feelings again. And the long weekend is making me even more anxious.

Are there any "tricks" you know of that helps reduce this descent. Does something specific help 'snap' you out of it? I'm really looking for answers from those that actually understand these feelings, but i'd be grateful to most any suggestions.