In a new post to his blog, Chris Hartjes shares one thing that you can use to make your code easier to test - using a dependency injection container and how it compares to a service locator.
People often ask me what's the one thing they could do for their code base RIGHT NOW that will make it easier to test. To me, the answer is simple: make sure you are using Dependency Injection (yes the link is long and has code samples in Java, but whatever). Without the ability to "inject" your dependencies into your code (whether it is class methods or functions) you will have problems testing modules of code in isolation from each other.
He shows the possible uses of DICs, including code samples, and talks the differences between the two. He explains that the real difference in them is how its being used. When it's used to add and remove instances, it's a container. When its actually put to use and passed into a class, it morphs into a service locator.