I found that brass wool works better at keeping tips tinned. A sponge removes all the solder and you have to quickly tin the tip immediately after wiping, otherwise it oxidizes. Wiping with brass wool leaves a thin layer of solder on the tip.

Also keeping the tip at moderate temperature range helps avoid oxidation - most manufacturers recommend to never exceed 400 C. JBC recommends to not exceed 370 C.

Hope this helps.