While the SharePoint setup wizard will happily and randomly select a port number for you, for SharePoint Central Administration, we at itgroove prefer something a little more managed, obvious and easy to remember.
We put Central Administration on port 23646
Why 23646? Because it spells out Admin on your mobile phone.
This was something I picked up from a guy a long time ago. Can’t remember his name now though or I’d share the reference, but he was a drummer too. Drummers are awesome.