Buying Back Intellectual Property

I’m a contractor who has (on the same project) recently switched from being a subcontractor (through an agency) to being a direct contractor with a client.

In negotiating a new contract, I’m considering terms which would enable my company to retain the rights to the software that is being built under the contract. I would like to know of terms that others may have negotiated in this vein, and what compensation they have provided to the client (e.g. user support, bug fixes, minor feature changes for a period of time? how long? was it necessary to buy back any of the work with cash?