Sometimes I'll use clear silicon depending on what I'm attaching. But mostly I use hot-glue. Hot-glue provides a really good hold, but if you need to remove it from a playfield plastic you can make it pop right off with no damage of any kind. Also hot-glue has the benefit of hardening real fast, using silicon you have to wait all day for it to dry.
By the way pinballbrat, you're making that WWF look very nice.