Use songs as a signal that it's time to transition from one activity to another. You can either select a specific song to play on a CD player, you can make up your own song or you can sing a song that specifically intends to indicate a transition. For example, when it's time to clean up one activity and start another, you can sing the clean up song:
"Clean up, clean up;
Everybody, everywhere.
Clean, up, clean up;
Everybody do your share."
Explain and model exactly what it is that you wish children to do when they hear these songs and then practice singing them and transitioning from one activity to another. When you play these songs, children will know what is expected of them and won't be shocked that it's time to transition.
Use an advance notice warning system to indicate to children that a transition will soon be taking place. Announce a warning two minutes and then one minute before transition. Use a clock or a stop watch to help keep track of time and when the one minute turns to 10 seconds, begin counting down out loud from 10 to one. Upon completion of the countdown, children should have cleaned up their current activity and be ready to move to the next one.
Clapping can serve as a means of indicating a transition from one activity to the next. When the time arrives that you want toddlers to switch activities, loudly clap out a pattern. Upon hearing the clap, children should stop what they are doing and repeat the clap and give you their undivided attention. You will likely need to clap your pattern a few times before you have everyone's attention, but with each series of claps, you'll gain more and more children's attention and eventually have all of their attention. Once you see that all eyes are on you, tell children what it is that you expect them to do.
Young children are often captivated by magic and all things associated with it; use this to your advantage when it comes to transitions with toddlers. Decorate a wooden dowel or an empty paper towel roll to look like a magic wand -- use paint, glitter, faux gemstones and ribbons to create your wand. When it's time to transition, pull out your wand, wave it in the air and recite a silly rhyme, such as "Rise and shine; it's clean up time." You'll grab children's attention and be able to tell them what it is that you want them to do next.