Easy. My glue stick.
I use an Avery brand glue stick for piecing.
Discovered it when reading "Landscape quilting" by Zieman/Sewel for placing raw edge pieces onto a background. Decided to try it for piecing and LOVE IT.
The benefits of the glue stick over liquid glue: 1) I don't have to iron it first to "set" it and 2) If I lay the piece down wrong I can pick it up and move it easily (as long as it hasn't sat for so long that it dried).
Does not gunk up the needle or the machine. Dries completely flat.
And yes, I did first test to make sure it would wash out cause sometimes when I'm laying down a line of glue along the seam I might go outside the 1/4". I also use my old iron so I don't gunk up my new iron (not sure that it will but not willing to find out!).
There are still times when I need pins. If I'm trying to match two particularly seams that need a little easing in the glue stick isn't enough.