It is almost always a dirty tip. I have done a lot of soldering, and have found that a bit of brass sponge works perfect, I have also used steel wool just fine. I like using nickel tips, but copper tips work just as good if you want to keep cleaning and replacing them. For lower heat guns I have been known to coat some copper (or nickel coated tips) tips in silver, and can tell you that is probably one of the best methods.
I use 60/40 alloy solder for most things, (I buy it by the 5 ib rolls, and silver bearing solder (I only use that when I want a really tough joint) by the .5 to 1 ib rolls).
Even with rosin core solder you need flux, I like the ruby paste, it works great for most things.