Linking algebraic expressions, words and pictorial representations
By the time I teach linear equations, I've already used algebra tiles quite a lot with pupils to create algebraic expressions. After working through one-step equations and ensuring pupils understand the processes involved (rather than just solving "by eye"), I move on to linear equations.
Not sure what algebra tiles are? Start here:
This was the first lesson I taught using algebra tiles (you can find out about them here) and it's my go-to way to introduce completing the square now. It's worth starting with a little work on factorising first, just so you don't completely blow their minds.
I discovered algebra tiles last year when, in a fit of desperation, I took to the Internet to find a good way of teaching completing the square in a way that might actually get it to stick in the heads of my Year 11s.