Ah yeah I did a little bit of experimenting last winter, here's what I learned:
1) Just like here on the forums, the best way to get attention is to get a discussion going. Reddit is all about conversation, you want your post to be the thing that starts it.
2) Post in subreddits that aren't related to comics, but are related to YOUR comic. I tried posting in /r/horror for example.
/r/comics and /r/webcomics work best if you you have short-form content, like a gag a day thing or a minicomic. /r/altcomix is interesting but they only allow self-promo on wednesday, also if your stuff isn't sufficiently 'indie enough' you'll prob get ignored (my stuff is weird and I barely got any attention there). There's also /r/comiccrits which is more for getting and sharing advice.
I decided to not use reddit for self promo anymore tho because the results were kinda eegh - Had a few new people check out my work but no long-term readers. Also - consider what @graveweaver said too. (I didn't know about that thankfully I gave up on self-promo before that happened) Will probably make the rounds next time I have a new minicomic, but I won't regularly promo my main series there except for maybe big events like it finishing a chapter (my chapters are long).
Ask yourself "What do people use this subreddit for" and then see if you can fit into that use. Kinda goes for any online community honestly. I would avoid things that explicitly for self promotion though. Cuz the only people there are other people who are trying to self promo - not people interested in finding new stuff to read.