I'm a litigation attorney. My priority system has to be pretty flexible. Here's what works for me:

To begin with, anything in the "this needs to be done by this date or I lose my job" category, whatever you want to call it, is published for me on a firm-wide calendar maintained by paralegals. That part is easy. The trick is to decide how to deal with everything else.

I try to respond to phone calls, letters, and email the day I get them. Longer letters, like a settlement offer or a detailed letter about a discovery dispute, may take more time than I can give them that day to finish, but I try to get them started so that they can go into my work in progress stack, which is a pile of plastic folders at the top left of my desk. Something that I was doing even before I read about the formal system mentioned in a number of places in this site. The plastic folders have added bonuses in a litigation firm where you need to keep documents with exhibits, prior drafts, etc. and hand them around among secretaries and other attorneys.

In the binder with my timesheets, also on my desk, I also keep a handwritten list of undocketed things to do that I am committed to doing soon or am in the process of doing (phone calls that I need to prepare for first, etc.) but that don't have an associated work in progress folder. Think of this as "active tasks without a folder."

This list is for phone calls, email, discussion agendas, and research that I will report on verbally. I try not to use it for collection of other tasks, but if I am in a rush and end up doing this I interpret the entry as "start of folder for x" or "enter x in your main todo list."

Basically, I have two "contexts." Writing (including letters) and other forms of communication (including email).

A list of things that I plan to do but don't have time to do right now are kept in my main todo list, which is a text file, with the entries in a form that is based on the codes that attorney's use to keep their timesheets, like this:

Jones pp ltr opp counsel re settlement Smith pp br msj [hold]

(pp = prepare, ltr = letter, br = brief, msj = motion for summary judgment, opp = opposing)

This is easy to sort by client, and the billing codes, in addition to being fast to type, also make good grep candidates if I'm in a GTD mood and want to, say, knock out some phone calls that I've been putting off (tcw = telephone conference with, em = email).

You could put dates at the front or back of the line (depending on how you wanted the list to sort), but I don't find soft dates especially helpful. I try not to make work product commitments more than a week out, two at the most, so it is manageable to keep track of this in my active work flow.

This may work better for me than for other people because so much of my work has hard deadlines associated with the litigation schedule, though.

I make a point on a weekly basis of scanning through the whole list and bringing some things into my active workflow, especially things that I have been putting off. The trick is to keep the "active, in progress, immediate commitments" category narrow enough that I can prioritize and reprioritize on the fly.