I'd like to write my sketches so that I can either build/upload them using the Arduino IDE, or optionally using GCC and a makefile.

I know about including the function declarations at the top, but is there anything else to do in order for my sketch to be considered valid C++ by my compiler?

[edit]

Understanding what the Arduino IDE does to .ino and .pde files is fine, but extraneous to my question, so this is not a duplicate. What I want to know is "how do I write a program such that it is considered valid both by the Arduino IDE and g++.

The official(?) makefile available here explains what to do if using the makefile instead of the IDE:

# The Arduino environment does preliminary processing on a sketch before
# compiling it.  If you're using this makefile instead, you'll need to do
# a few things differently:
#
#   - Give your program's file a .cpp extension (e.g. foo.cpp).
#
#   - Put this line at top of your code: #include <WProgram.h>
#
#   - Write prototypes for all your functions (or define them before you
#     call them).  A prototype declares the types of parameters a
#     function will take and what type of value it will return.  This
#     means that you can have a call to a function before the definition
#     of the function.  A function prototype looks like the first line of
#     the function, with a semi-colon at the end.  For example:
#     int digitalRead(int pin);

...but this doesn't explain how use both the IDE and a makefile. [/edit]