Why routines?

to avoid duplicate code: copy and paste is a design error
to reduce complexity
to produce code with strong, functional cohesion: doing one thing only and doing it well
make a section of code readable

Layout

seperate routine parameters by one space
seperate routines by at least one blank line
routine names: first letter lowercase, first letter of each following word uppercase. Underscore is forbidden!
unresolved: add a semicolon add the end of a function (after the closing curly brace). This would enable us to use Edwards packing algorithm (thanks to User:Marc Jansen for the hint)

use a strong verb followed by an object, like printDocument() or checkOrderInfo()
good names describe everything the routine does
make names as long as necessary
avoid meaningless names (like handleCalculation() or performService() etc.)
maybe use a description of the return value
in classes, don't include the object name again (document.print() and not document.printDocument())
establish conventions for common operations (avoid mixtures like id(), getId() etc.)

> 200 lines: understandability suffers

sort order: input parameters first, then output parameters
if several routines use similar parameters, use a consistent order
don't use routine parameters as working variables
> 7 parameters: readability suffers
check values of all data from external sources / all routine parameters (garbage in garbage out is not an option: either "garbage in, nothing out" or "garbage in, error message out")
- decide how to handle bad inputs (robustness vs correctness)
- anticipate divide by zero errors, or division by integer

minimize the number of returns in a routine, but return a value immediately if it improves readability
don't use recursion if you can use sequential code (Fibonacci etc)
create simply routines for simple purposes
move complicated boolean tests into routines
add a notice to deprecated functions
make sure that you don't use operators as functions! For example write

if (typeof myVar === "undefined") {

instead of

if (typeof(myVar) === "undefined") {