Try Microsol decal softening fluid, a small bottle will last years. There's two sorts, a red and a blue bottle, the red is the stronger
IIRC, but has been known to dissolve some decals, though I've have no trouble with
GW ones, but it does eat Tamyia ones.
Also decals adhere better to a gloss surface than a matt one, and you don't get the silvering effect on the clear parts of the decal either.
So, gloss varnish the shoulder pads (leave overnight), then soak the decal in the Microsol fluid. Then apply the decal as normal. Once fully dry (give overnight) paint over the shoulderpad in Humbrol dull coat, and you'll never be able to see the decal lines, it'll look like it was printed or painted on.
For Black Templars I made little cuts into the decal between the arms of the cross before soaking, but you won't be able to do this with circular decals. If you do cut a decal, I'm told you should use scissors as it makes the edge of the decal curl down slightly. Cutting up decals with the knife makes the edge of the transfer curl up slightly and it'll want to peel on the model.