Gaining backlinks can certainly be one of the most frustrating aspects of any search marketing campaign. In the old days, when Googlebot was a mere child, incapable of rational thought, it was easy to obtain backlinks. Everyone just threw up a links page (and trust me, many of them looked like someone had "thrown up" on the page) and traded reciprocal links willy-nilly. Alas, as Googlebot gained in intelligence, those free-trading days disappeared into the links netherworld.
These days, we have to actually create content worth linking to! Oh my, what a concept. Still, even with excellent content, links are hard to come by. That's why I suggest doing a little link bartering now and then.
Link bartering is not simply the act of trading links with others. Instead, it is the bartering of services that may naturally lead to links. The key is that links should not be a required aspect of the service trade, but that getting a backlink from the trade would be at least somewhat likely, based on the fact that it would just make sense to do so. Let's take an example to show what I mean.
You notice via one of your social networks (Twitter, a forum you visit often, etc.) that someone has a problem that you can help with. Many of these "problems" are minor, and as such, you should freely give your assistance simply because it is the "neighborly" thing to do. Nothing should be expected in return for this type of help. On the other hand, occasionally, these problems are bigger, and it would not be un-neighborly to suggest some sort of service trade for these issues. The idea is that you would help resolve their problem, and in return they would trade some sort of service for you.
The type of trade might involve service-for-service (such as coding work for design work, for example), but it may also involve something more along the lines of service-for-review. In this instance, you would help the person out, in return for them doing a review of your site, service, widget, or product. In my opinion, it is important that you place no restrictions of any sort on the review. They don't have to give a favorable review. They don't' have to link to you. If they do link to you, they can nofollow it if they choose to. Assuming you have confidence in your site/service/product, you probably won't have to worry about getting a bad review - and you probably won't have to worry about not getting a good, solid link out of it. Obviously, it's best if your widget, product, or article is quality. That gives you the best chance of getting both a good review and a good link to it.
If all goes well, and your social neighbor writes a good review for you, be sure to socialize his or her review yourself! Let your social networks know about it. Encourage retweets to it. The person who wrote the review will appreciate the traffic, and you've also upped the chances of someone else writing about it and linking to it as well!
If each of you are happy with the services you've supplied each other, there's a good chance the two of you will naturally trade services again in the future - simply because it makes sense to do so! That's how it works in the "real world" and it works equally well in our virtual worlds. Be neighborly, give freely when you can, and trade services when appropriate to build strong friendly foundations that can result in backlinks now and in the future.