I think Just In Time reminders are the best way to deal with this
blog.codinghorror.com/the-just-in-time-theory
The key thing to consider is when to intervene, e.g. when are there points of negative action? Some examples could be ...
first time you press a key in your first reply (if you are replying because Someone Is Wrong On The Internet xkcd.com/386 )
replying over and over to the same person
replying very rapidly to the same topic
etc
I also feel that downvoting is a very dangerous thing to add to any community, so be careful with that. It's OK on Stack Overflow because those are systems of data, facts, and science -- things that can usually be verified. But in Discourse we're dealing with opinions, so downvoting isn't appropriate.
It is interesting how the conspicuous absence of upvotes can sometimes feel like a slap, though ;)