I think that resurfacing the rotor is the best bet. I have sold all of the over the counter brake squeal remedies, and few work in rare occasions. It comes down to harmonics. Sometimes the silicone type can absorb enough of the vibration to dampen the squeal, but it is most likely from having a glazed rotor surface. Douching your pads with brake cleaner may help, if there is dust a grit. Think about it this way, you are trying to create friction on a smooth surface (assuming the rotors have not been resurfaced). You need two rough surfaces to create a good amount of friction. If the rotor surface is smooth, the pad has little to bite into and "skips" across the surface of the rotor. But it does this at a high enough rate to create a harmonic effect, and hence the squeal. here's another way to lok at it. By putting new pads on an old rotor, you are also asking the pad to conform the the uneven surface of the rotor. So you have these very small areas of the pad surface that don't contact the rotor, pockets of gas for, expand rapidly to due to the heat caused by the friction. The pockets of gas force the pad off the surface, so the gas can escape, and then slap down again. Vibration leading to harmonic squeal.
The other factor may come down to pad composition. Organic pad material is usually softer, and has a different coefficient of friction than a semi-metalic pad. So if you have semi mets, changing to a softer organic pad may absorb more of the vibration and decrease the squeal. But, the real issue is still not being addressed.
So why do ceramic pads tend to be quieter? Because they have such a high coefficient of friction that they essentially resurface the rotor face. They are much more abrasive than semi-mets and organics, so expect to replace you rotors more often, or replace them with high quality rotors.