You can do this activity with your students at the board or on worksheets.
Provide them with a really long horizontal sum. Something like this:
3 + 11 + 7 + 5 + 5 + 4 + 9 + 1 + 2 + 13 + 8 =
The idea here is to group pairs of numbers into tens or multiples of ten so that they can quickly and easily find the sum using mental math.