Gnome on Ubuntu comes with an included “Remote Desktop” feature (vino) that allows you to VNC into your existing desktop session (usually on the :0 display). However, XFCE does not have a native VNC package built in. The best option I have found is to use vino, the same VNC app that Gnome uses. Here’s how:

Install vino:
$ sudo apt-get install vino

Configure vino:
$ vino-preferences

Then enter this command in your Autostarted Apps to start vino server:
/usr/lib/vino/vino-server

This works well for me, the only feature I have found that doesn’t work is the “disable wallpaper on remote login” setting.