One of the best ways I've found to start building relationships with students (please keep in mind I did this with elementary students) was to ask them for songs that made them feel good BUT there couldn't any inappropriate lyrics. I would sometimes use instrumental versions if I couldn't find clean versions of the songs. I would use this to build a playlist and when students were doing independent work, I would play it. Students loved it and got to share with me why they chose the song.
When I first started teaching, I had all types of catchy phrases to get students attention (elementary school students loved it!). A few years ago, I read something that changed my direction when it came to this. Instead of trying to get their attention by them responding in some way (verbally or with an action like clapping), I say, "if you can hear my voice, take a deep breath and let it out slowly and silently." Most of the time, after once (maybe twice) of saying this, students are quiet and have slowed down their nervous systems with a deep breath. Even when we're working independently, I will do it. Just to give students a chance to breathe deeply and give their minds a break. Have you tried this? What else do you use to get students attention that has worked well or what have you used to give your students a mindfulness break?