Remember how I like to have students recite the monthly poem aloud during transitions? Another thing I like to do is sing skip counting songs. Just like reciting the poems, the students know they should be to the carpet or in line before the song is over. It's also a good way to pass the time while lined up waiting to go to lunch or out to recess.
The skip counting songs I use:
2s to the tune of Yankee Doodle to 30 (and sometimes 50)
3s to the tune of Are You Sleeping? to 30
4s to the tune of Row, Row, Row Your Boat to 40