If you back up the private keys (to paper or otherwise), then the recovery process will be to install a new wallet client, and import the keys. Most of the wallet clients have their own instructions on how to import private keys, and the nice thing about this is that a private key from one wallet can be imported into just about any other wallet.
So, if your chosen client were to stop being supported, you could just import the private keys into the next client you choose.
The reference client (Bitcoin-Qt) makes it pretty complicated to import private keys (and the last time I used it it the import was a bit buggy). If you plan to use the reference client, then you can find the "wallet.dat" file (where it is stored depends on the type of operating system you use) and make a copy of the file that you store somewhere safely. To recover this backup, all you have to do is:
- Shutdown Bitcoin-Qt
- Replace the existing wallet.dat with the backed up one (might want to make a copy of the existing one first "just in case")
- Start Bitcoin-Qt