In our math centers, we have been focusing on adding and subtracting two and three digit numbers. To help my kids remember the steps, they reference these posters at our centers.
Since students need different levels of support, counters, number lines, and pictures are used to help students understand the concept. In particular, I found that using a 0-20 number line was very helpful for my students who were having a hard time. The other number lines featured can be useful if you teach students to estimate their answer.