With the agency I run, we use a flat fee (a monthly retainer), based on the size of the client. Some months you will work a lot more, some months a lot less but if the ROI is there for the client, they will be happy with paying the fees.

It all comes down to ROI and predictability with clients - I think that as long as they know what to expect each month and you deliver, you are a lot more likely to have long-term clients.

The #1 issue I see in getting paid by the hour or day is that you should deliver quality and not quantity - it's so counterproductive for both parties to clock in the hours on something that involves creative work and it can work against both.