When I introduce completing the square, which is after factorising quadratic trinomials, I give them a quadratic trinomial that doesn't factorise easily, like [math]\displaystyle x^2 + 6x + 1[/math] and tell them that if they can figure out how to factorise it, they're much smarter than I am, because the only things that multiply to give 1 are 1 and 1, or -1 and -1, neither of which add to 6.
Then I say it would be much easier if it was [math]\displaystyle x^2 + 6x + 9[/math], because then it factorises easily to [math](x + 3)^2[/math].
Of course, we can't just turn [math]\displaystyle x^2 + 6x + 1[/math] into [math]\displaystyle x^2 + 6x + 9[/math] because they're not equal. But we CAN write [math]x^2 + 6x + 1 = x^2 + 6x + 9 - 8 = (x + 3)^2 - 8[/math].
I then remind them of the difference of two squares rule, and point out that the first lot of "stuff" is clearly a square, and that the final term can be written as a square if written as [math]\displaystyle \left( \sqrt{8} \right) ^2[/math], and so can be factorised with DOTS.
Then I leave as a task for THEM to figure out how to find the "missing" term to create the first square, i.e. to complete the square. They nearly all get it for themselves.