Flux is for woosies. There I said it, LOL.
It's not for woosies, it's for people who haven't spent their lives soldering every day, day in, day out. Soldering for most of us amateurs consists of once in a blue moon, to once a week or so. Flux makes it easy for novices to solder better. It promotes the solder to flow onto a surface. They help to keep out impurities like oxidation caused from heat and they help with wetability.
Rosin core solder has flux in the center. It's rosin flux and solder together. Some of it is a preference, as some will use solid solder and flux, while others will use rosin core solder.
The most common reasons for poor soldering are dirty parts that are being soldered, too little heat and too much heat and oxidation from keeping the solder hot too long, as the piece being worked, will oxidize from heat.
Look at the melting point of the solder you are using. You should have the heat of the iron hotter than that point and a lot of it has to do with how well the iron will keep the heat at that point. I think 60/40 should melt around 400F. The iron can get hot, but if the wattage is very low, it isn't able to stay hot when you touch something and the part you are soldering accepts the heat faster than the iron can handle. That's why I recommended a 40 watt or higher iron.
The iron should be "tinned", meaning it should have a very thin coat of solder on it, not a blob. When I tin a tip with solder, I use a paper towel and wipe the excess off, or I use the Radio Shack stuff and just stick the tip in it.
Hold the iron to the part you want to heat. Test the solder onto the part till it melts on to the part, not the iron.
Am I wrong here guys?