For me it's Post-It notes! Stick them in books for reminders. Stick them on my laptop. Or make them the basis of a lesson. This week I used them for the basis of forming a scatter plot:

To start this activity I wrote a few basic instructions on the board to the effect of:

• measure your height at the back of the room
• get a post-it note from Mr R
• draw a cross on it
• plot your point on the board

Now I did this with a group of kids that hadn't seen a scatter plot before, and without my help (I actually moved myself away from the board so I couldn't help) they managed to grasp what needed to be done without any fuss and just get it done. The great thing was that for the few who were unsure, someone else explained how it worked.

So, 5 minutes into the lesson, I have a scatter plot of real, student data on the board, and have started a discussion about outliers (prompted by one 13 year old having a size 11 that I hadn't factored in!) 

In past years with this activity, I have started with some kind table of points etc... and 20-30mins later we have our first scatter plot on graph paper. This activity worked that well that I used the last 10mins of the lesson to do a similar plot for height vs arm span (requiring only one new measurement).