2. Don't install any shared data (such as factory presets or other resources) in a directory that is only accessible by the user who installed the software (e.g. %APPDATA%, %USERPROFILE% or HKCU on Windows). It's a PAIN for those who know how to use the operating system correctly (ie, having a dedicated non-privileged user account). If really necessary, copy that data to the respective user's folder on the first start of the plugin.
3. If applicable, make the path to any data folder configurable.
4. Don't assume the plugin directory is writable when the plugin is used.
Addendum: make your plugins 64 bit. Seriously. It's 2013. It should be your top priority for this year

