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Dave,

Remember that you ask a question here you will get MANY answers. So, with that said, here's mine.

I was NASA certified to solder at one point in my life. I too have had over 35 years of soldering. Clean work parts is a very important part of the equation. I use alcoho to clean the parts to be soldered.l However, the materials being solder are also important. For example, copper wires solder easier that "Tin" wires. A general rule is that if the wires are "gray" in color it will be more difficult to solder and usually need a higherr wattage. Flux is very important but also comes in the solder quite often. A lot of time (soldering copper wires) this is fine. Heat distribution is also very important. Usually, the tip is placed FLAT against the metal(s) to be joined. Here's some important information regarding Flux. Pay particuliar attention to the last section.

In a soldering process, heat is applied to the parts to be joined, causing the solder to melt and be drawn into the joint by capillary action and to bond to the materials to be joined by wetting action

Capillary action, is the ability of a substance to draw another substance into it

'Wetting is the contact between a liquid and a solid surface, resulting from intermolecular interactions when the two are brought together.

The purpose of flux is to clean surfaces that are going to be joined together to enhance wetting by solder in the molten state. To accomplish this flux must be able to deoxidize metal surfaces at high temperatures without decomposing.

It is important to remove any contamination from a completed job regardless of where it comes from, flux or other processes. Residues left on a solder connection can cause electrical shorts or corrosion.