1. Renogiate with your employer: but beware, many employers with this clause in it are unlikely to remove it, plus even bringing up the possibility will make an independent thought alarm go off in management/your HR file. I myself do not seriously suggest this strategy. It also makes it almost impossible to then implement strategy 3, as record of the request is itself something that may link your future work back to this company.
2. Quit/find another employer.
3. Lie. Work on your own project somehow, and make sure nothing can concretely link it back to your time working with said company. Do this until you can implement strategy 2.
http://www.nolo.com/legal-encyclopedia/who-owns-patent-right...
When you have some strong leads, you approach your employer about renegotiating that part of your employment contract. Don't threaten to quit or hold those leads over your employer's head. Express that some of your colleagues with other companies are explicitly allowed to own all of their projects developed during their own time.
If that fails, follow through on those leads. Don't work on any personal projects until you have officially left your current job.
If that isn't an option, at least be head-hunted. Then you can negotiate much of the contract.
Edit: As far as I know this is a very standard clause in tech company employment contracts, and is perhaps the legal default even in the absence of such a clause. For example, see "Employed to invent" under http://www.nolo.com/legal-encyclopedia/pre-invention-assignm... .
California is an exception to the above: here employees may retain the rights to IP they create on their own time, not using resources of their employer (including company laptop), and, importantly, not in the same line of business as their employer. In the tech field, the "same line of business" caveat can be killer.