The above picture is a newspaper clipping of my uncle, Dudley Doerflinger, receiving a Teacher Recognition award by the Glendale Chamber of Commerce. I've only ever heard amazing things about his classroom, his creative lessons, and his ability to hold the attention of a child. He retired before I was old enough to see his classroom.
While I've seen and heard of methods ranging from music clips to hand signals, my current favorite one is clapping. My favorite method is to do a 1-2 clap with your hands and the students repeat the 1-2 clap in response. Once they do the 1-2 clap, the students are to be quiet, keep hands still, and look at the teacher as they await for instructions. Those instructions might be to clean up, line up, prepare for lunch, sit on the rug, etc.
If the teacher does the 1-2 clap and all students do not respond in a 1-2 clap, then the teacher repeats the 1-2 clap and the students again respond with a 1-2 clap. Any students that do not respond in the 1-2 clap after the second round of clapping is given is given a consequence like changing the color on the color chart, staying in from recess for a few minutes, or whatever your behavioral method is.
I observed one fabulous kindergarten teacher who used a bell for center transitions. The bell sat on her rolling cart that she used to hold her supplies and books as she did small group work, etc. When it was time to switch centers, she would ring the bell. Sometimes she would have the student that put forth the most effort during center time to ring the bell.
I remember being in kindergarten and when it was time to come in from recess, the line leader for the day would use a tambourine to gather everyone inside the classroom. The line leader would march around the yard (we had a separate yard from the rest of the school) and everyone would "line up" to march inside the classroom. Kids loved being line leader and being able to play the tambourine. It made you feel so special.
Get other ideas for transitions by reading all of the linky party posts at Little Miss Kindergarten!