I use Microscale's Micro Set and Micro Sol on most builds and those two products alone allow me to lay most decals onto a model with reasonable success, but when I need to apply a decal over a particularly complex shape nothing beats Solvaset. I don't know what's in this witch's brew, but I'm convinced it's not of this world. This stuff is great. The bottle I have has been in my toolbox for more than 25 years!
I had to use Solvaset this weekend on my 1/72 F-15B. The unit tail bands at the top of the vertical stabilizers lay over the ECM blisters on the trailing edge of the vertical stabilizers. I knew Solvaset would work its magic, and it didn't let me down! (That said, if you look carefully you can see some issues with my application of the decal for the tail code. Ugh!)
If you don't have a bottle of Solvaset on your workbench you really need to get a bottle, or two.
If you're in the United States you can get Solvaset from Sprue Brothers. For my UK readers I'm told Hannants stocks it but I don't see it listed on their web site.