1) Don't do mods other people are doing -- it is stupid, pointless, and wasted time.
2) Don't modify the functions if you don't have to -- instead, write a function that does the job, and call that function.
3) Don't screw with Admin.php much
4) Don't screw with Instant Messaging much -- a new version is coming some time soon
5) Don't screw with Packages.php or PackageGet.php --
MINE -- bugs? TELL ME!! -- MINE

6) Comment you code! Readable code is a GOOD thing, not a bad thing!
7) before your modification code, put this:
// Custom Stuff Mod by John Doe
// starts here

after you modification code, put this:
// Custom Stuff Mod by John Doe
// ends here
9) INDENT (either 2 spaces or a tab)
//This is nice
if($name == "Howdy dood") {
print "Hi there";
}
// This isn't
if($name =="Howdy dood) {
print "Hi there";
}
10) Let us developers know what you are doing
11) If a new version of something is coming out, wait before screwing with that something
12) Don't build on top of other mods
13) Put information in the "modification.txt" file
14) Put PHP queries in the "modification.php" file
15) Don't use the "modification.sql" file -- it is screwed up currently
16) Put mod stuff in the "modification.mod" file
17) Upload/download those four files as ASCII text, upload/download the mod file (mod.mod.yp) as BINARY
18) The current tags supported by Package Manager are:
<file>
<search>
<before>
<add>
<replace>
19) Don't make stupid, pointless mods
20) Try to fit several bug fixes into a single mod