Applying custom decals to plastic figures I'm having trouble getting decals to stick on action figures. Decals can be a decent alternative to painting, as they take much less skill to apply.

But I find certain edges or corners won't stay put, possibly from touching oils on my fingers, first. 

How can I get those stubborn decals to stay? Is there a liquid or adhesive that would work? 

The decals are foil-based, so they're essentially waterproof, if that matters.
In my scale modelling, I use Microscale Industries' Micro Set and Micro Sol which works great for clear decals.  These products increase the adhesion and slightly dissolve the decals which allows them to conform to curved surfaces better.  Once they are dry, I sand any visible decal edges very lightly with a 600 or 1200 grit sponge.  Finally, I repeatedly cover the whole model with light coats of an acrylic clear coat, building the coating up until the decal edges disappear.

I'm not sure if the Microscale chemicals will work with foil based decals, but the edge sanding and clear coat should help them stay where you put them.