It really depends upon the topic. Social media is a great tool, and better for some markets than others. It's difficult to ignore the opportunities for a new market/industry until it's proven not to work. I use social media regularly.
"How to" videos make worthwhile additions, too. Viral videos work in B2B and B2C.
I regularly add content to keep the sites fresh.
Link development is a regular activity, but only quality links. I really don't care about a link for the sake of it.
Anything newsworthy is scheduled into a news release program.
I use traditional, offline media, in which is often tough to get coverage. Because it's tough, the link/coverage is valuable.
e-mail newsletters are very useful where there is a valid opt-in list. Issuing e-mail newsletters to bought-in lists is close to spamming, so I don't use that method to get to new business.
Once a site is established I hate to touch anything if it's ranking well. And, I never optimize a site just for Google or Bing.
Recommendations are probably the best ROI of everything. Satisfied customers tell their friends, and then their their friend's friends, etc.
Customer support: This is the most important of all as it's far more cost-effective to retain a customer than it is to gain a new one.
Oh, and this sounds crazy, I know, but I send out snail mail letters. It's worth spending time researching the contacts, and writing to them as individuals. It's as if snail mail is ignored by so many, giving opportunities for those that use it.
If you try to use Reddit for the sake of site promotion, you're generally doomed to fail. Especially if your Reddit post history consists solely of submissions involving one website.
If I browse any of the new queues for Reddit, I downvote anything submitted by an account with almost no post or comment history save for submitting links to the same site. If a Redditor has multiple submissions for a site, and doesn't seem interesting in otherwise participating in the community, I ask the admins to blacklist their domain.
Now if you owned a successful automotive blog, made comments in various threads, and submitted the occasional link to your site in an auto subreddit, that could be successful. The only exception is when a business (GoPro is notorious) uses shill accounts to submit videos for viral marketing purposes.
Depends on what the niche is. For my own sites, heavy on the Facebook, plus I maintain contacts with all the local news media via Twitter, Facebook and email. Some of my enterprises have t-shirts and other items, all of which have my URLs on it.
For clients - again, depends on the niche. My biggest has published an 84-100 page catalog once a month for like 30 years - they didn't stop once they went online, they just went from sending 15k a month to 125k a month. Plus they have cultivated around 100K opt in email addresses for a weekly email blast.
I have B2B clients doing well with LinkedIn, and everyone does well with PPC if I'm the one managing it.
My brother works as a chef for a local restaurant, and he and I are planning to set up an Instagram account for the place so he can shoot pics out from the kitchen. Another eatery I know tweets out "code words" and if you get there within half an hour with the code word, you get a free something or a discount.
Still other clients have put their products into Amazon, figuring if you can't fight the beast, might as well join.
I've partnered up with other sites and other companies to co-market stuff. Been asked to guest blog, but I just don't have time for it. But done right, it can be a good tool. If you're local, there are usually all kinds of places you can hook up with to either share costs or trade services or reciprocate marketing.
A significant number of my clients are local (250 mile radius) and what seems to generate more interest than anything is direct mail (snail) community by community and radio and tv. Works a treat, and for one clent with a more unique product, ended up going national and has some growing interest internationally. Those last two expansions came from Bing and Google over the last two years.
In all cases, it is not overnight, or even a few months. It always takes time to get the word out (not the SE's, the CUSTOMER FEEDBACK!) and site/company branding.