Not my own tip but a handy one considering some of the limitations of this phone.
Samsung BlackJack II Tips for MP3 Ringtone (Get Rid of 300KB size-limit). WMExperts has come up with an interesting workaround and registry edit that allows you to not only set any MP3 of your choosing as a ringtone, but it also gets rid of that annoying 300KB size-limit.

Yes, Now you can use any MP3 file, of any size, as your ringtone!

Here’s the ‘Step by Step’ to do:
1. Save your MP3 to your Samsung BlackJack II’s memory (I like to put my files in “My Documents”).
2. Use your BJ2’s “My Stuff” file explorer (there is the standard Windows Mobile File Explorer and the “My Stuff” file explorer - use the latter) to navigate to your MP3 file.
3. Highlight your MP3, hit “Menu” and set that MP3 as your ringtone.
4. The “My Stuff” browser will ask you if you want to move the file to your “My Sounds” folder - do it. The file gets copied, not moved, to “My Sounds” so your original MP3 file is still chillin’ in the “My Documents” folder.
5. Now to edit the registry to nix that300KB file size limit:
- Go into HKEY_Current User > ControlPanel > Sounds
- Delete the key of FileSizeLimit
6. Done, You should now be able to set any MP3 as your ringtone.

(via WMExperts)