Code that goes into Gnome uses the Linux kernel coding style (which is basically the GNU coding style but indentation is 8 spaces, and braces are on the same line as the statements that open the block). Make yourself familiar with both coding styles (the GNU coding style and the Linux kernel coding style)
Try to make your code readable in 80 columns
When you change things, edit the ChangeLog, so the other people can know what you've did. The ChangeLog is an important document, as it is shipped with the package and it lets the end user and programmers not familar with the GNOME way of things to know what has been the evolution of a package. It also allows people with no continuos network access to find out what changes were done and by whom
When you're writing code that should be written better add FIXME comments so it's easy to recognize.
Try to use defensive programming techniques: the glib library provides various utility macros that help you make your code more robust. g_return_if_fail, g_return_val_if_fail, g_warning, g_error and g_assert are your friends.
Document your programs.
If you do changes to the APIs documented here or add some new ones please update the documentation.
Add translations for your native language. We would like have a full internationalized GNOME. (so if you know more than one language do translations for all them) (we would like all the idioms, from francais to old greek :-) )