Here I am again, complaining about other people's programming style... This time it is more about the general graphical design of applications.
1)Do not treat the PSP as a desktop computer. I have seen lately loads of applications which pretend to be window managers, or so-called VSH-replacements. How come almost all of them adopt the desktop-like feeling of windows/linux computers? :/ The PSP got no keyboard, no mouse, no big screen, and not much memory. So why give it the "look" of a real computer?
Sony realized this and made a VERY simple to use VSH, the "XMB" as people call it.
The PSP is still a gaming machine, no matter what anyone can say. If you need a laptop, just get one... They're now quite cheap... (The Sharp Zaurus is kewl :P) As an example to this, one of the recent apps I saw lately is "PSPExcel". Coded in LUA, with an awful style, it has no reason to exist. I repeat: if you want to do Desktop stuff, use a normal computer. The PSP is for GAMES. What are the most successful homebrews? Games or stuff enabling you to play games (emulators).
2)Do not use copyrighted names/graphics in your apps. You will hear this all the time, but probably simply ignore it. If you're too bad at CG, ask someone to draw you stuff, or get a model under CC... There are quite a few examples of this in so-called homebrew... A real dev does not COPY/PASTE stuff, but creates it. (Those of whom I'm talking will recognize themselves)
3)Think about UI and user-interaction when you make an app. That is, don't ask the user to press "L-R-Left-Select-O" just to enable something ;) And DO NOT use the debug font to print stuff on the screen! (I think that battery plugin used it...) There are free font libraries which you can use, and one which is very dummy-friendly by IWN ( http://insomniac.0x89.org/index.php?id=46 ).
I guess that for now it's done ;) Think about the stuff I wrote before posting an answer. Oh, and, this thread is being posted in Dev Dungeon to avoid script-kiddies polluting the topic too much.