I do clean it often - after nearly every pass.
I saw that tutorial before and I think that maybe my iron is too hot for what I'm doing? Or am I pressing too hard with the tip??
you're only using a 40watt iron...it's not too hot...in fact, maybe it's not hot enough and you're pressing hard because you think that might help?
What type of solder are you using?
How often do you tin the tip?
What flux are you using?
I'm trying to think of anything that might be different from how I solder that might lead me to an answer.
I've been soldering for so long, that it's just second nature to me, so I can't even go through it in my head to see what else might be happening.