Give yourself as much leverage as you can get over their deployment cycles. Use both carrot and stick - Include features they can't live without with the hardes

Give yourself as much leverage as you can get over their deployment cycles. Use both carrot and stick - Include features they can't live without with the hardest of deployment maintenance burdens, and make sure you have tight adherence to your software lifecycle - Every question about 1.0 should be answered with "We don't know, 1.0 is out of support" from the second that it's EOL date passes. Include the currently running version in every logfile, if not every log statement. Make it clear to you where they are and run your own run-behind that upgrades only after they do. This will be painful, but it will save you overall time.