Web Rings were one of the methods that actually helped an older version of this forum to get off the ground. It was very effective at the time, but I'm really not sure if web rings are as popular or common today.
Find a good one and you're likely to get at least some additional traffic from it.
To a great degree, web rings don't exist any more. At least not in the form they took during the mid to late 90's. Web rings aren't really a good thing these days. They're much closer to directories/link farms in the eyes of search engines than they are to so called information portals.
Reciprocal Links: No
Three way links: No
The thing about directories is that they don't hurt, but they don't really help all that much either. There are very few directories that exist where it's actually very beneficial to have a link from them. The only one that really helps you at all is the DMOZ, which has been losing popularity with the major SE's anyway.
Both Maxbloom and Vonnagy have very good points, and having spent a lot of time worrying about search engine optimization (even to the point of writing a search engine and spider from scratch (Desert Realm Fantasy & Scifi Search Engine), I can hopefully shed some insite.
1. The big search engines have a large staff that look at all of the ways people try to manipulate their results. They update their algorithms often based on their findings. Thus, what is good today may not be good tomorrow.
2. Search Engines account for aproximately 1/3 of all of the traffic to my photo community site, which gets the most traffic of all of my sites so I'll use it as an example. At the time of this writing, it has about 600-700 visitors a day. (Certainly not a ton, but I've done it by myself without spending any money on advertising). The rest of the traffic: 1/3 direct traffic (people type it in directly to their URL or use their favorites links), 1/3 from referal links.
3. There will always be trendy things. Look at webrings... they were the trendy thing for a while, but now they are considered spammy by some. Unless you can devote a lot of time to promoting via trendy stuff, stick to the basics.
4. The basics include:
a. Good content - smart search engines look at your content and determine how relevant it is. This includes good grammar, good content, etc. Smart users look at your content and determine if you know what you're talking about. If you have bad grammar or the content isn't relative, they will hit their back button and go somewhere else. If you have good content, they'll save you to their favorites and come back again.
b. Links - Links are essential whether you care about search engines or not. If you trade links with somebody, you'll get a few visitors from them and they'll get a few from you. This might not be the flavor of the day for search engines, but it's still free traffic to your site. Yep, good one way links are essential, but there's really only 2 ways to get them. 1 - have awesome content that compells somebody to link to you (see part a above). 2 - pay somebody.
So what is good content?
It's relative information that a person is looking for, presented in a way that is easy to get to and easy to read.
Good information guidelines:
1. New Information added daily if possible, but at least weekly such as articles, photos, etc. Can be anything, but you need a reason for people to come back (remember 1/3 of my traffic is from direct visitors and most of them have been on the site before). This is why 'blogs' are popular today. It has nothing to do with it being a 'blog' - call it whatever you want. It's simply a place where information is updating regularly.
2. Information needs to be layed out well. People don't spend a lot of time reading pages, instead they skim through pages. Use headings, etc to aid those skimming through your pages.
3. Information needs to be something the person was looking for. Keep it about the same topic. If your site is about dogs, for example, an article about Bush's latest political statement probably isn't going to matter to your users unless it's also related to dogs somehow.
4. Information needs to contain your keywords, but they need to be included naturally. If you were writing a paper for your photography teacher, you wouldn't include the word photography 74 times. However, it should show up a couple times along with other related words. Maybe something about a photographer, a photo, a picture, or an image. Not only do these help make your page more relevant, but they also help you show up in search engine results for the other words.
These days it helps to have an off-site blog where you can add content on a regular basis. Provide a backlink to your main site and the search engines will love this because it's regularly updated. Add your blog to a few blog directories to help the blog itself.
Would like to stress here, One of the best ways to promote your site is to be listed in Google, in first page of results is Prime.
Many people pay good money to be first in results (pay others to get them there) others spend years barely being in results if at all.
The next 2 things were listed in the OP's but seemed to be very over looked and under utilized and in my opinion the most critical.
1, create a Sitemap.xml (manually or online tools)
2, Go to Google webmaster tools, create an account, verify it and Upload the site map.
Google Spider will be there much quicker.
Having good content that appeals to Spider is the next most important step, along with having good In & Out links.
Keep in mind, that Google Spider seems to have an Attitude, don't get upset (I have seen many upset/hurt or just plain PO'd) just keep trying until you start to get listed.