How do I advertise my comic?
Project Wonderful
This is what I use. It's been the best so far, because I can bid with any budget, from free and up. Also I've setup ad boxes for other people to bid on, so I can earn ad revenue to fuel the ad campaigns I'm placing on PW. I still have yet to put my own money into the site, I've just been spending what I've earned.
Also a benefit of placing a free ad is the ability to test out a site and see how many clicks it gets before bidding money on it. I've done this with a bunch of site and have friended all the ones that do well for me. This way I can go to my "Relationship" page and bids on all the sites listed knowing they are effective sites for me to advertise on.
Facebook
I've tried advertising on Facebook, which is a complete waste of time and money these days. Facebook is a terrible place to advertise, and here's why:
When you pay to advertise your page to get more likes, most of the time it gets sent to Like farms. So you'll get a bunch of meaningless bloat Likes and very few people that are actually interested in your page.
Why are bloat likes bad? Facebook's algorithm is based on the total likes, so when you post something only about 5-10% of the people that Liked your page will see it and you're relying on those 5-10% to enact with your post for it to reach more of people that Liked your page. (messed up system) So if the 5-10% are just people from Like farms, they won't interact with your post, it'll get ignored, and never gain traction.
Facebook is useless for promoting your comic on! You have to promote your FB page on other sites to get genuine likes, and if you're going to promote anything it should be the comic rather than its social media page. Unless you're gaining something, like Patreon support and/or Tapastic ad revenue, promoting the comic site is top priority.
I haven't done paid advertisement on Twitter yet because I don't want to bloat my accounts like FB. Plus, I see Twitter as more of a networking place for now.
Google AdWords
This takes lots of practice and learning to master, also you need a big budget. I got a free $100 coupon to try it and it barely lasted a week. You can set daily budget limits and control how much you can spend a day, however, in order to compete with other bidding ads it's difficult to bid less than $5 a day per ad.
Also with Adwords, you need to carefully consider niche markets and relevant keywords to ensure it's reaching the most people possible that are in your target audience and be interested in your site. And to know all of that, it's super important to study your comic site's analytics. It's complex. I'm still learning how to swim in this deep end of the swimming pool.
Sites like Adwords is a bit difficult for free webcomic creators to get into at first because you need the budget to spend on it, so you can earn ad revenue to buy more ads but also make something back. It's very catch 22. I recommend practicing/mastering Project Wonderful first before tackling this beast of an advertising playground.
Those are the few I've tried advertising on (since it was cheap to promote on), however, I've heard great things about buying ad space through Comic-Rocket and TCL. I haven't tried them yet because they cost money to place an ad with. ($50 and up)
Reading your original post again, I think you mean how do we self-promote rather than advertise. There's a big difference as you'll probably see with my answers.
Self-Promotion
Now self-promotion on the other hand, is the best form of advertising I can do for my comic. It's how I build my reputation online, It's free, however very much time consuming. There's a ton of clear answers on how to effectively self-promote here on the forums. (I posted more than my fair share on this several times)
Pretty much anything and everything I do, online, in the name of my comic, is self-promotion for me and my comic whether directly or indirectly. Directly by posting page updates on social media, creating ads, banners, and graphics promoting us at a convention, debuting issue in print, merch, Patreon, Kickstarter, video trailers for the comic and so on.
Or indirectly. By indirectly, I mean anything I do online that isn't for the sole purpose to get people to me site. It's basically being genuine, which is an appreciated way to self-promote. I do this by networking, participating in online communities, sharing tips/tricks/advice with others, participating in collabs and art events, interacting with other people's comics and leaving a genuine comment, promoting other comics, gushing my love for another comic in a guest comic, fan art, WIP sketches, Art Request Livestreams for fans, so on. This forms builds your reputation as an artist in the webcomic community. Use it wisely.
If things are fuzzy while reading them on the forums, feel free to ask. It's hard to given all detailed answers all the time. 
