How to communicate an increase in the yearly maintenance license?

I am selling an enterprise system. So far there has been no real sales-volume, so I do not have any price model to publish. The few customers I have are really happy with the solution.

My question is, how can I communicate that I basically want more in the early maintenance fee (which is their only expense at the moment). Should I try to come up with an obscure technological reason, or not come with any reason at all, or maybe use some other arguments?

I should say, that I do not have a contract with any of my customers, they basically just pay invoices and we have an understanding (I do not want to discuss the good and bad points about this).

Sell an upgrade and increase maintenance with the upgrade. Changing maintenance fees without an upgrade can leave a bad taste in the customers mouth. Just ask SAP or Oracle.

I don't have a good answer, but I'll give you an anti-pattern.

One of my clients just got one from another vendor. It came off really poorly talking about the economy, their poor management of costs, and said that going forward it would be tied to the rate of inflation (much like a COLA). They also mentioned that there was a pass through licensing cost that was going up -- this was the only one that didn't sound like an excuse.

Basically it came off as "we want more money and you're stuck so you're going to pay it."

Explain why the price is increasing,
Give advance notice of the increase (atleast 30 days),
Lock in previous pricing,
Take the initiative and offer alternatives,
and make it easy for the customer to implement the price increases.

If they get value for money they would not mind to much. If they do not feel they are getting value for money raising your prices are going to be a hard sell.

Ask them how they feel about your service and try and do some comparisons to similar type of systems. If there is now real alternative for your clients and they are content then you can increase the price easily.

By rolling out price increases gradually you can also see how your customers react. Start with a customer at a large company that you know is happy.