What are your best practices for making sure newly hired developers quickly get up to speed with the code? And ensuring developers moving on don't set back ongoing releases.
Some ideas to get started:
Documentation
Use well established frameworks
Training / encourage mentoring
Notice period in contract
From a management perspective, the best (but seemingly seldom-follow) practice is to allow time in the schedule for training, both for the new employee and for the current developer who'll need to train them. There's no free lunch there.
From a people perspective, the best way I've seen for on-boarding new employees is to have them pair program with current developers. This is a good way to introduce them to the team's coding standards and practices while giving them a tour of the code.
If your team is pairing averse, it really helps to have a few current diagrams for how key parts of the system are structured, or how key bits interact. It's been my experience that for programs of moderate complexity (.5m lines of code), the key points can be gotten across with a few documents (which could be a few entity-relation document fragments, and perhaps a few sequence diagrams that capture high-level interactions).
From the code perspective, here's where letting cruft accumulate in the code base comes back to bite you. The best practice is to refactor aggressively as you develop, and follow enough of a coding guideline that the code looks consistent. As a new developer on a team, walking into a code base that resembles a swamp can be rather demoralizing.
Use of a common framework can help if there's a critical mass of developers who'll have had prior experience. If you're in the Java camp, Hibernate and Spring seem to be safe choices from that perspective.
If I had to pick one, I'd go with diagrams that give enough of a rough map of the territory that a new developer can find out where they are, and how the big of code they're looking at fits into the bigger picture.