Second system syndrome is a software development pathology that often happens when a piece of software is developed in two phases. The first release of the software includes everything that's relatively easy and quick to implement. The second version includes the rest. Unfortunately some of the features in "the rest" turn out to be unfeasible. This can lead to the second version never being completed. Time goes by, the seasons change, but the software release is always a year away.
The best way to develop software is to admit from the start that there will be many iterations and that not all features will make any particular release. First you make a priority list consisting of the features you want in order of importance. You then implement this list, making sure to revise it as new information becomes available. You then release your software on a fixed schedule.
The fixed schedule is useful for making sure you ship something. It changes the question from "What are we going to do?" to "What can we do by the next release?". This is subtly important because in software there are things that will take an extremely long time to do but nothing is really impossible. Estimating how long something will take becomes harder the longer the time span and the more complex the problem. If a new feature is really complex then you can find yourself on a project that will takes years to complete. What is more, complexity feeds on itself, causing schedule ships and increasing the amount of confusion about how to fix problems. Software can become a hellish tar-pit of intense pressure and slipping schedules.
Making the commitment to release software on a fixed schedule turns the problem on its head. Instead of shipping a product late because one single feature is horribly behind schedule you instead focus on trying to accomplish the most important things you can before the next release. This essentially guarantees that you'll always have the most critical features in any given amount of development time. If a feature is behind schedule it will miss the release date and get automatically pushed back to the next one.
If you release on a fixed schedule make sure that you're aggressive about taking features out of the release that they aren't ready. There needs to be a clear and enforced policy that if a feature isn't ready on time then it's not going to be in the release. This means that developers have to ensuring the feature they are working on doesn't break the codebase. Invasive new features need to be written in such a way that they can be turned off or otherwise disabled if need be. Frankly, clearly separating new development is a good practice in all cases since it helps QA test new features or bug fixes without getting mangled in a new feature. Modern revision control tools like Git and Mercurial can be a help here with their branching features.
The most common complaint when trying to implement rapid releases is that some feature is vitally important and it's only a little behind and so the release should be delayed just a little bit. This is not only the thin end of the wedge but things are rarely only "a little delayed". What I've seen is that the release gets delayed to include this "vital" feature but several others are added in since there's now time to do them. Next *those* features get delayed slightly so more features are added to fill in the gap. Then those get delayed.. It can turn into Zeno's project management. The release is always just a little bit in the future and the goal of a release every month turns into a single release in one year. What you're actually doing is you're delaying all features for a single feature. It's not a nice thing to do to all your customers whose features are actually shippable.
Being strict about the cutoff date stops feature creep, it allows customers to get features sooner and it increases software quality.
My favourite benefit is that development teams no longer rush to meet an unrealistic deadlines and by skipping on testing. If development is running late they can take their time and do it properly because their feature can hop on the next release. It also remove the temptation to commit the sins of self deception; passing off unfinished software as merely buggy, for example. The effect of this one is an endless QA cycle as developers use QA as a sort of to-do list generator. "I'm done! And on time too! Oh, there's a bug? Ok I'll fix it. At least I was officially 'done' on time.". Yeah right.
If you release often enough, say every four months, you don't need to create maintenance releases for existing branches. Any bugs can be fixed on the trunk because customers will have that code in good time. Additionally, it's less likely that you will introduce a regression because less has changed since the last release. If there's a really critical problem you can still ship a maintenance release but it's rare you'll need to do this in practice.
When it comes to testing, adding automated unit tests and regression test becomes more important in rapid release software. Since the codebase is always changing it's important to not constantly break things every release. Automated unit test and regression tests is a best practice to avoid unmaintainable software. Rapid releases just make the consequences of unmaintainability more dire.
There are a overheads associated with creating a new release of the software. Most of these should be automated anyway. Those that can't like documentation updates and manual QA feature testing should be easier with short release cycles since less has changes since the last version of the software. This means less to update and test.
Version 4 is out? Yeah, whatever, everyone knows that version 3.4 is the best. Nice laptop by the way.
Another potentially annoying aspect of releasing on a schedule is it's hard to make a fuss about the new version of the software because major features are done incrementally. When I used a rapid release cycle on the Myster project this turned out to not be a problem. What happened was that we were releasing so often people would be visiting the website regularly to see if there had been a new release. I released on a monthly basis and the public realized the Myster was constantly being updated an improved. A majority of our user base upgraded every time we released a new version. And we didn't have a auto-update system! Having rapid release cycle communicates to customers that you care about issues and new features and can fix them quickly. It also creates a constant background buzz. We found ourselves on the front pages of many a news web site every time we released - once a month.
Releasing often and on a fixed schedule does mean that your marketing team has to think of the product more as a continuous stream rather than a single specific version. It doesn't stop you from selling the features of the new version but it does mean you should direct people to the latest version and not develop a brand around a specific release. If anything, I'd considered creating a brand around a specific version of a piece of software a marketing anti-pattern. It means you have to compete against your own software's older version every time you release. How silly is that?