Flux, flux, flux.
Flux is good for everything, and on everything!
When you wipe the iron, you expose bare metal. The tinned surface oxidizes quickly, and relatively deeply. An iron left on overnight this way can be challenging to bring back to brightness; left on for a week, potentially hopeless!
Try to put fresh solder (metal + flux) on the tip, and knock off the excess, before resting the iron for some time.
The excess of solder also provides some squishiness beneath the crud (which will now consist of oxides and charred flux), so there's something to wipe off next time. A freshly wiped surface has little liquid under it, and no sacrificial/protective film of flux.
Try to keep idle times down. If you foresee a long time, turn down the temperature. (Sadly, this is painful with the Hakko FX-888D, when it was but a touch on the analog model. The competing Weller WES(D)-51 has always had the dial, however.) Likewise, when you need some big heat, turn up the dial. I occasionally use up to 400-430C on mine, but only for minutes at a time (the tip gets seriously crunchy after a little use at temperatures quite that high!). Almost all my work is done at 300-350C. Keeping it on the relatively low side (like 300Cish) also helps when you forget to turn it down or accidentally leave it on -- 16 hours at 300C is a lot better than 16 hours at 400C!
If you aren't constantly soldering, part after part, your tip will get dirty, and you will have to use extra solder. For the infrequent use I typically see when building prototypes, I would guess I use twice as much solder just cleaning the tip, as actually ends up soldered into the board. Don't be stingy. Solder is a cheaper and less bothersome consumable than tips are.
I also find differences in flux. Subtle, but enough to be a bother. I never liked Kester 44. I think it burns too quickly. Likewise, the stuff even in brand-name flux pens -- it pushes away from the heat, the solvent boils and spatters, it's not active enough, and it chars quickly. The best solder and flux, in my opinion, were from Radio Shack / Tandy: the solder feels just slightly better to use, and the paste flux (at least from one particular vintage) has some sort of glycol base which seems to stick around much better than anything based purely on rosin. Just active and mobile enough to cleanse the joint, while remaining passive as a protective film.
There are probably others with similar formulations and use. I recall using an old tin of acid paste flux (obviously, not for electronics purposes) which was some combination of petroleum jelly, rosin, zinc chloride and ammonium chloride: smooth enough to wipe on, stays in place, very active obviously, and fairly easy to clean up.
Tim