I have used a thin smear of Elmers glue, which works well. But you have to carefully work out all the bubbles of glue, until there appears to be nothing left, and then you have to let it dry for a week or two.

I have also used simple syrup (equal measurements of water and sugar by volume, bring to a boil, let cool). It's easier to work with, but the bubbles are harder to see. Again, you have to work out all the bubbles, and until it dries the decal will be fragile. It will get tougher in a day or two, but you can't be sure it's really dry for a week or two.

Both of these water based adhesives allow the decal to slide around until you have the location right until you start pressing the glue out. Once the glue is really dry, the decals are durable. Even so, clearcoat helps.