At the end of a session or end of the day, to get children to tidy up effectively and efficiently i have a "tidy up song" which i play and the children have to have the classroom tidy before end of the song.

I have a chart with my name on one side and children on the other. If the children clean the classroom before the end of the song they get sticker on their side of the chart, if not the sticker goes to the teacher.

At the end of the week, I add the stickers up and give a small reward.