On my PalmPilot, I'm really only using couple of these fields, which I've renamed to Password, Birthdate, Account #, and Misc. I use the Birthdate field in conjunction with the Birthdate application from Fahl Software. While a lot of folks will say they prefer to just keep birthdays in the Date Book application, its always made more sense to me to keep them here in the Address List with the other information you might have for that person. The Birthdate application takes this information and will then place entries in your Date Book for you.

Of the other three custom fields, I really only use the Account # field, for the records I keep of banks (checking account, etc.) and credit card companies. I don't keep Passwords in my Address List anymore, as I now keep those (and my actual credit card numbers) in Secret! (Secret! is an application by Andreas Linke which keeps that info encrypted). And, I've found that instead of using the Misc field, I prefer to enter miscellaneous information in an attached note.

Finally, after having sung the praises for categories above, I've got to admit that I really don't use them in the Address List application. Sure, I have them set up and place records in their appropriate categories, but I find that 99% of the time, its best to have the application come up in the All "category" and then use Look Up to quickly locate an address or phone number. If your Address List is coming up in the last category that you selected, you can turn that off (and always start in the All category) by un-checking "Remember last category" box in the Preferences dialog.

Date Book
For the Date Book application, I prefer to not see any extra lines when I don't have appointments for them. As has been covered in many forums, this can be done by setting your Start Time and End Time to be the same time (set in Preferences).

I keep holidays in the Date Book, although you can also place these in a note in the Address List application and have Birthdate maintain them for you. The main reason I like to keep holidays here instead of letting Birthdate do it for me is that all the holidays can be set up as repeating events, which lets me avoid having to verify the list that Birthdate uses. Just remember that holidays like Thanksgiving that are on the 4th Thursday of the month can be entered as Monthly repeating events (every 12 months) and set to repeat by Day (which will show as "The 4th Thursday of every 12th month)... which works!

If you choose to keep birthdays in the Date Book, you might want to remember when you add them to add a corresponding entry a week or so ahead of time to remind you to buy a gift and/or a card.

To Do List
I don't really use the To Do list as much as I used to, because I've found that if I need to do something on a specific date (which are a large number of my To Do items) I tend to forget to check there. Instead, I add these "have to do" items as un-timed (or even timed if I'd like to have an alarm associated with them) events in the Date Book. A nice feature of PalmOS 2.x is that you can set an Alarm on un-timed events, which will display that event the first time you turn it on the day its located on.