The modern, read/write web thrives on processes which accept and coordinate a large set of contributions from a set of contributors that is almost as large. Before this was common on the web, open source software capitalized on the aggregate efforts of communities. However, not every web 2.0 site or open source project succeeds in attracting or retaining contributors. In the case of web sites, this is often due to poor user interface or other highly visible problems
Software projects, however, typically suffer from a different problem: the cost of creating and integrating contributions. This ailment also afflicts proprietary software, where it is actually rarely diagnosed, and even more rarely treated.
I argue that all substantial software projects should strive to minimize two variables:
- Aggregate cost adding a contributor 
- Maximum individual cost of accepting a contribution
If either of these variables are not minimized, a project will fail to fully utilize the efforts of its key members, never mind begin to capitalize on the long tail of contributions. High contribution costs waste countless millions of hours of productivity.
In a company, the easiest way to measure the cost of adding a contributor is to measure the time from signing the employee agreement until the moment that the software builds and runs on the new-hire's development machine. Don't forget to count this in man-hours because this will surely require the efforts of a manager, a more senior engineer, or both.
Additionally, you should also count the time before the new developer feels some level of confidence in the codebase. Can this new person dive right in and start hacking on a new feature? Could they fix a bug without everything falling apart? How many times will they need to knock on the door of the expert? 
Measuring the cost of accepting a contribution is subtly different. The aggregate cost is not of critical importance as long as it is justifiable given the benefit of accepting a contribution. However, minimizing the aggregate cost of accepting a contribution is already the goal of any successful software project. Teams are always striving to be more efficient, but it is a local optimization. If you want to increase the number of contributors and contributions, you need to minimize the maximum cost to maintainers, newcomers, other team members, and communities.
As any good manager knows, they must suffer the pain of meetings, accept frequent context switching, and act as a shield from politics. They must do all of this to ensure their reports are as productive as they possibly can be. Similarly, senior engineers should donate some of their time to analogous tasks in component and systems ownership. Although these tasks introduce a cost which causes an individual contributor to spend less time actually contributing, it opens the door to greater net contributions.
So here come's the concrete advice part of the post...
Some ways to reduce the cost of adding a contributor
- Buddy builds - One button should instantly and automatically allocate a fresh machine, enlist in the source tree, build without additional dependencies, execute all tests, and provide detailed status and results.
- Standardized tools - Allow contributors to transfer their skills in version control, issue tracking, building, testing, and communication. Every team is different, but the more you can reasonably share, the more you simulate one cohesive team.
- Encourage branching
- Understand that contributions can come from anyone 
- Keep documentation up to date and complete - Hold new project members accountable for the correctness and completeness of the newcomers wiki page.
Some ways to reduce the cost of accepting a contribution
- Automated unit tests - Insist on a high code coverage and a low flakiness rating. Require a 100% pass-rate before committing to the main branch.
- Explicit coding standards
- Code reviews
- "Lieutenants" - Put an "owners" file in each directory of your source tree. Fill it with the usernames of those who should perform the code reviews for those files and sub-directories. Keep these files up to date.
- Widely and clearly communicate project vision and direction - This ensures misguided contributions will be rejected swiftly.
- Regular release cycle - Do I really need to say "release early, release often"? Releasing often enough will force you to reduce the cost of your release process as well.
Got any more?
 This post was prompted by my
startup hobby project's failure in this respect. Adding a third team member took several hours of my attention, but should have taken several minutes.
 Sorry Stephen!
 Some discussion lists at Microsoft are notorious for responding to internal feedback with the URL http://career/ and possibly a snarky comment. These people are artificially setting the cost of adding a contributor to be a career left-turn and the cost of accepting outside contributions to infinity. I hope these dinosaurs go extinct.