Here are some random tips I've come across with my Sony Ericsson K608i phone. Many of these will also work with other Sony Ericsson or other Bluetooth phones. For example, Telstra are now offering the K600 which is essentially the same phone.
Firstly Mac users should head over to mobile.feisar.com for all of the files to use with the phone, including better instructions for iSync than I provide. Great site that I should have mentioned sooner (sorry!).
Secondly, if there is a file type you are frequently sending to your phone (such as MIDI or AMR audio), on your Mac you can set the application to launch them to Bluetooth File Exchange. Do this by selecting the file, pressing Command-I, and clicking Change All..., then you can double-click and hit Enter to send the file.
On the phone itself you can make Notes permanently visible by using the Show in standby option. This is very handy for to-do notes. First write the note in the Notes menu (under Applications) and Save it. Then in the notes list, select More and select Show in standby.
Make use of the shortcut menu! This is accessible by pressing up on the joystick while in standby. You can save a lot of keypresses with this. For example, I use this for the Select network (which is otherwise buried) menu item, giving me back a feature which was only one button press away on my old phone.