There are a lot of hacks out there that allow you to create a customized format for the date and time in the menu bar. While many of these work, they are a lot more complicated than they need to be. The absolute easiest way to completely customize the date time in the Mac menu bar is to use iStat.

iStat is an application that gives you little menu bar widgets that allow you to monitor your Mac. While it's a great program for monitoring, it also has a less-advertised feature of also being able to show the date time in the menu bar. And the amount of customization it allows is as much as you are going to get anywhere. There are scores of pre-defined formats, you can drag and drop format elements to create your own format, and you can use the "programmers" date time format elements (%B, %d, etc) to create a date and time that is exactly how you want it.

If that's not enough, you also get richer information when you click on the date time in the menu bar. It shows a monthly calendar that you can scroll through as well as shortcuts to iCal. Here's how it looks:

iStat date time widget details

All this and it's free!