Are you using V4? If so, then you should use the new events instead. Take a look at this article for a sample:

http://umbraco.org/documentation/books/api-cheatsheet/using-applicationbase-to-register-events

You could do something like this (written directly here, so may have syntax errors):

public class AppBase : umbraco.BusinessLogic.ApplicationBase {

    public AppBase() {
        Document.New += new Document.NewEventHandler(Document_New);
    }
    void Document_New(Document sender, PublishEventArgs e) {
        //Do your businesslogic here
        string yourvalue = "blahblah";
        
        //This will set and save the value to the document
        sender.GetProperty("yourproperty").value = yourvalue;
        
    }
}

This fires when you create the document. So that is before you save/publish or enter anything into the doc.

Principle is the same if you want to do it every time the document is saved/published. See a list of all events here:

http://our.umbraco.org/wiki/reference/api-cheatsheet/using-applicationbase-to-register-events/overview-of-all-events