I never relinquish ownership -- that's nonnegotiable for me. The concession I'm talking about here is that the contract specifies that I retain ownership and copyright, but I commit to offering my employer the option to exclusively license it from me for a limited period of time. What the limit should be is open for negotiation, although I prefer that the limit isn't specified in the employment contract, but that it gets negotiated should the employer actually want the license.

When the exclusive period is up, that doesn't mean the license is inoperative, only that I am free to license it to others as well.

Note that when I say "I" here, I really mean my attorney. These sorts of things can be tricky, and I am no lawyer. Also, it's very hard to make this sort of thing foolproof -- if an employer wants to screw me, there are a few ways that they could.