Ok after it is tinned and you have taken the excess solder back off with your tissue use your clean wet sponge and the tip should be somewhat shiny although it may dull slightly if the heat is up high (this speeds up the oxidation but dont worry it is only the solder oxidising and not the tip if you have tinned it). If it gets dirty from flux that really isnt a problem. when working for any length of time that is going to happen just clean it as you did before with solder, tissue then a clean wet sponge. when done soldering clean you work with an acid brush ( i cut the bristles down a bit to "stiffen them up") soaked in alcohol then blot dry untill its clean and shiny. Alot of times I will use my tissue soaked in alcohol to clean the tip before I use the sponge. This seems to help keep the flux residue off. The contanimates that we wory about are oils and dust/dirt they keep the solder from flowing properly
hope this helps you out