In my case the flame trap improves the idle, because I do it at the same time I clean the throttle body. But I'm sure it's really the throttle body that improves the idle situation. Getting at the flame trap is so much easier with the TB off, so it makes a lot of sense to do these two things together. I do it every couple of years, at the same time I flush my coolant (just for rhythm's sake; there's no connection of course between the coolant job and the others).

Cleaning the flame trap depends on how dirty it is. If it's not clogged up with a lot of gunk you can clean it up nicely and easily with carb cleaner. If it is clogged, just replace it. Also really check all of the various hoses connected to it, especially the skinny one coming off the side. Replace any hoses if you see the slightest sign of cracking or clogging.

If you use synthetic oil you'll never have to worry about your flame trap. It's really only when dino oil stays in the engine too long that you see it clogging up.