While there are many ways of reducing compile time in Unity, like moving your code into DLLs, I recently stumbled on a very simple approach that most developers can apply immediately and see results from right away.

Create a folder named Standard Assets and move code that does not change often into it, such as plugins, shared core systems, or other stable support code.

According to Unity's documentation:

Scripts that are placed in the first group ("Standard Assets", "Pro Standard Assets" or "Plugins"), will take longer to compile, since when they are compiled the third group needs to be recompiled too. Thus if you want to reduce compile times, put scripts that seldom change into group 1 and scripts that change a lot into group 3.

This guidance came from the Unity 4.1.2 documentation, but I timed the effect myself in Unity 4.3.4f1 and saw the benefit directly. After moving stable code into Standard Assets, compile time dropped by roughly half, from about 9 seconds to 4.5 seconds, in a project with more than 1,500 C# files.