Extra IT services we can offer to existing clients

Hi. I have a selection of small to large IT clients and provide desktop, server and other network related IT support to them. As its quiet work wise I want to develop other income streams to bring in more work. My question to you is – what other types of work can I offer my clients related to the IT servicing field which they will want but don’t know it yet?

For example customers tend to react to problems when they occur not be proactive in ongoing maintenance. Has anyone found that occasional printer cleanings is something customers want and so on.

Your ideas please.

Thanks
Bruce