This was the only thing that had me really flummoxed to begin with, but luckily google and various Xbox web sites came to the rescue and pointed me towards the free Mac app called 'SharePoints'. After scanning through the manual I proceeded to setup my first share - it's really easy to do and within seconds my Mac's Downloads folder was being shared using the SMB protocol. I won't go into the full details of setting up SharePoints as it's really easy. However there's more to it than that, as a configuration file on the Xbox needs to be edited so that XBMC knows where to look on the network for your shared drive.
You need to edit the file called XboxMediaCenter.xml (it's in the main xbmc folder) and add a section similar to this for each share you've setup with Sharepoints:
For all the information you'll need on this go to this page or alternatively this page - they helped me enormously! Now when I browse for files in XBMC as long as my iMac is on and SharePoints is running, one of the items in the list is 'iMac' and by clicking on it up comes the contents of my Mac folder :)
So far I've only setup the one share, and not touched the iTunes streaming facility. I've copied music and photos to the Xbox via FTP and all the media functions of XBMC work brilliantly. One thing I'm really pleased with is being able to watch TV recorded using my El Gato EyeTV 410 - I just export to Toast .mpg format as usual, but instead of writing DVD-RW disks, I just drop the files into my shared video folder - superb.
Stopping your Mac from sleeping while streaming
Rather than constantly tinkering with your Energy Saver settings, it's much easier to install the brilliant Jiggler, which can be set to 'jiggle' the mouse whenever you like to keep the Mac from sleeping. I set mine to only jiggle when Sharepoints was running and it works like a charm.
An alternative to SharePoints
If you don't want to use SharePoints you can use symbolic links instead - I've not tried this myself but here's an article about it.
Setting up XBMC's weather display to work in the UK
Depending on what skin you use, XBMC can display the weather forecast for you! The code for London is UKXX0085 which is as close as I could get to where I live. Annoyingly the MC360 skin doesn't display the weather anywhere so I switched to the Project Mayhem III skin and tried it from there. As you can see it works like a charm.
RSS Feeds
Some skins like Project Mayhem III can display scrolling RSS feeds - this page has info on setting this up, which I've not tried yet.