How do you bring it up when you start dating someone new?

Great question, and you are not alone with this. Catching the trigger early is where most of the difference comes from.

Honestly I just wait until I actually like them, then mention it casually like it's no big deal, because it isn't, and the ones worth keeping around have never once made it weird.

I usually just say it early and casually, like "heads up, I get cold sores sometimes so I might dodge a kiss for a few days now and then," and honestly most people have shrugged and said "oh me too" or "no big deal." The dread of telling them was always way worse than actually telling them.

I usually wait until it actually comes up naturally, like if I'm skipping a kiss goodnight because I feel one starting, and then I just say it plainly and a little sheepishly. Honestly the people worth keeping around have always shrugged and said "oh yeah, my mom gets those too," so it turns out to be way less of a moment than I build it up to be in my head.