Before the launch of iCloud, sharing an Apple ID linked to iTunes or the App Store was straightforward -- you would simply log in to the same account across all devices and you'd avoid having to purchase an app more than once.
With the launch of iCloud, users are still able to share one main "iTunes account" (if you will) and still have their own iCloud account linked to the rest of the iOS services. Although, not many people realize how straightforward it is.
When initially setting up a new device, go ahead and log in using the device owner's iCloud account/Apple ID. For example -- if you're setting up the device for your wife and she has her own Apple ID, use that. If she doesn't, create one for her. Doing so will help keep your e-mail and iMessages separate.
During the process, the App Store and iTunes apps will also be linked to the same account. We're going to change that in a moment.
Launch the App Store. Make sure the Featured tab is selected on the bottom. Scroll all the way down until you see the account name.
Tap on it.
Tap on Sign Out.
Sign back in using the shared Apple ID.
Now the iOS device is linked to two different Apple IDs. One, is the device owner's ID and includes the iCloud services, the other ID is limited to just iTunes and the App Store.
Keep in mind, according to this Apple knowledge base article, once you register an Apple ID to a device (speaking strictly about App Store/iTunes), that device cannot be associated with another Apple ID for 90 days. Also, you can only have a total of 10 devices (both iOS and computers combined) associated with an Apple ID at any given time.
By logging into the shared Apple ID in iTunes/App Store you'll be able to install the same app across multiple devices, as well as keep all of the personal information synced via iCloud separate.