My most effective trick in helping students remember how to round and estimate is the integration of some of my unique songs, chants, and raps. The first song I created help students remember when to round up or round down.
Next I created a song to help students remember the steps to estimation. Students frequently want to add, subtract, and multiply first and then round their answers. The problem is that it could result in a wrong answer on a test. They need to remember to round first and then add, subtract, or multiply. You can hear the tune of the song when you purchase my Estimating with Addition and Subtraction PowerPoint Presentation at my TpT store. (I honestly could not figure out how to add the sound here and I made up the tune.)
Finally, I (with the help of a class a few years ago) created a song to help students remember how to multiply after rounding by multiplying by multiples of 10's, 100's, and 1,000's.
You can hear the unique tune of Zero the Hero and get so fantabulous teaching tools, tips, and lessons when you purchase my awesome PowerPoint presentation Estimation with Multiplication from my TpT store.
Here are some of my favorite videos and songs from YouTube and United Streaming that I use when teaching my units on Estimation: