Proactively work with your clients to offer them migration paths to hosted, pay-by-the-drink delivery models. I realize that this is heresy, but wouldnt you rather lead them somewhere than chase them later?
Build templates for migrating from internally hosted applications to externally hosted ones. These pre-packaged configuration management tools would go a long way toward building your credibility and keeping your customers happy (and just plain keeping them).
Embrace open source software. Sure, its the anti-Christ but its also the future. We all know that there will actually be proprietary open source flavors but the interoperability of these hybrids will be vastly superior to what we have today. You all need to think about becoming the Red Hats of your industry.
Publish all the APIs you have and support them. Make sure that theyre clean and secure and update them on a regular basis. Give them away free with a smile and keep them coming.
Redefine your relationship with your clients as partners not adversaries. If you think youre welcome in most enterprises then you probably thought U.S. troops would be greeted as liberators by the Iraqis. As you know all too well (perhaps after a few necessary glasses of wine) many of your clients really hate the vice you keep them in. The do not see you as a partner or a friend; they see you as someone trying to squeeze as many dollars as possible from their shrinking technology budgets. You really need to turn this around and fast, lest you lose more and more of your long-suffering clients to alternative software delivery models.
The real challenge is the reinvention of your business while you simultaneously cannibalize the business model that made you billions of dollars over the past few decades. I know, I know, its hard to do these kinds of things, but, trust me, the clock is ticking. If you fiddle too much longer, you will trip into a free fall that will cost you billions over the next few decades.