If you or your organization uses custom functions across several solutions, you'll likely want to develop some sort of centralized library of the functions you've developed. That way, when you find yourself in need of a particular function, you won't have to rack your brain remembering where it was used before. Also, centralizing the function library is a way of creating a knowledge base that can help your organization leverage its past work and can aid in the training of new developers.
Your library can take many forms. One option, of course, is to create a FileMaker Pro file for your function library. Minimally, you'll want to include fields for the function name, its parameters, its formula, and a brief description. You might also use a container field to store a sample file for a particular function. Another "nice to have" would be a related table for storing information about where you've used the function.
As of the time of this writing, there's no way to move custom functions from one file to another using tools in the FileMaker product line, although cutting and pasting formulas to and from the library isn't terribly time-consuming. Custom functions are, however, part of the Database Design Report (DDR) that can be produced by FileMaker Pro 8 Advanced. If you're handy with XML, or are looking for a fun first XML project, you might want to use the XML output of the DDR to create your function library.
You also might want to investigate FMRobot, a tool sold by New Millennium Communications that can automate many development tasks, and that includes the capability to move custom functions between files.
Finally, if you always want to have a particular set of custom functions in your files, create a sparse template file that has them in it. Then, rather than creating new files from scratch, you can just duplicate and develop on top of your template.
Part I: Getting Started with FileMaker 8
Using FileMaker Pro
Defining and Working with Fields
Working with Layouts
Part II: Developing Solutions with FileMaker
Relational Database Design
Working with Multiple Tables
Working with Relationships
Getting Started with Calculations
Getting Started with Scripting
Getting Started with Reporting
Part III: Developer Techniques
Developing for Multiuser Deployment
Advanced Interface Techniques
Advanced Calculation Techniques
Advanced Scripting Techniques
Advanced Portal Techniques
Debugging and Troubleshooting
Converting Systems from Previous Versions of FileMaker Pro
Part IV: Data Integration and Publishing
Importing Data into FileMaker Pro
Exporting Data from FileMaker
Instant Web Publishing
FileMaker and Web Services
Custom Web Publishing
Part V: Deploying a FileMaker Solution
Deploying and Extending FileMaker
FileMaker Server and Server Advanced
Documenting Your FileMaker Solutions