7
A - Get to learn the XOOPS API's (Database, users, modules,...). Also see S
C - CVS can save your life
D - Database: convert every database call to one using $xoopsDB
I - Include mainfile.php on every user page
P - Turn PHP debugging mode on
S - Read the source code of other modules (I liked mylinks a lot) to learn the API and basics of XOOPS modules (basics and more of course)
T - Try to use templates if possible
X - XOOPS Control Panel: very easy to integrate the original app's control panel in Xoops': just got to insert xoops_cp_header() and xoops_cp_footer() where appropriate
Got to get some sleep now, will think about this