Think interactive

One of the main benefits of email marketing is its quickness and the amount of information that can be used to optimize the way you communicate with your clients/prospects.
If the job is properly done you will not only increase drastically your ROI but you will also help your clients and increase their overall satisfaction.

Here's my point of view: If you have a mailing list that you broadcast once in a while with your latest products... fair enough, but you are probably missing the best part of email marketing: Interaction.

Let's take a few examples of what we do in Cabestan with some of our clients, I swear that these examples are very (in Cabestan at least) easy to implement.

Ticket Booking site:
If you have a ticket booking website (or any events related website) and broadcast a weekly newsletter with a selection of events, you are doing good but here is what we do with our events related clients:
Just add on your website a "place an alert" button on each result pages (especially when there are no matching results), this info would then be pushed inside the DB, and an automated daily campaign will be set to scan the latest events' RSS feed daily and send out an email to all users that have today's artists/events in their booking alerts list.
If you do so, you will not only add a new service for your consumers but you will benefit from one of the highest ROI I've seen on email campaigns.
NOTE: this can also be used on an eShop website with alerts on product availability.

Software retailer:
If you sell software or any product that has a limited life time, please, please do set an automated reminder campaign to be sent a few weeks before the licence (or product) comes to an end.
Just keep somewhere inside the DB the purchase date, the rest is just too easy...

Transform your prospects in clients:
I assume here that you are addressing your clients and prospects in a different way and not blast the same email to everyone.
If so (and it is important that you do) then you can still take it a step further.
As a matter of fact, clients bring you money, prospects cost you money. It's therefore quite obvious that one of your main goals as a marketer is to transform as quickly as possible these prospects in clients.
The best way I've found yet is to create a scenario that all your prospects will go through when they subscribe.
This is again very simple to do, all you need is to set a given number of touch points with your prospect until he becomes a client, for example:

1. Welcome email - D0
2. Voucher code (valid for 15 days) - D1
3. Voucher alert - D14
4. Special offer (like free shipping) - D30

Of course, should the prospect order something at any time, he is then out of the automated campaign and enters the clients list. This is just an example, you can do it your way.

This list of possible interactions is not a closed list, there are plenty of options and possibilities that will depend mainly on your needs, your business model,...

As always if you feel like you need some help, or if your ESP is not offering such services, you can contact me, I will be pleased to help you out.