If you have a large customer base and your customer information has been captured in a Database you may wish to consider bulk text messaging.
There are quite a few providers available and the cost of sending a text message to a client is quite minimal.
The more information or fields you have available on you clients the better - you can be totally flexible in the use of this application. You can decide exactly who is to receive the text message e.g. you may decide to only send a text message to you female clients or clients within a certain age group