1) You need to currently have the unlimited plan on your line.
2) You will need another line with an upgrade available (either a dumbphone, or one that already has lost unlimited data)
3) Go ahead and upgrade the 'donor' line and get the phone you want. If this is a dumbphone, add on the cheapest data plan available. Not the Shared Data plan. Use the 2GB option.
4) Now you have a new phone on the 'donor' line. Go ahead and activate the original dumb phone on that line. This will drop the data plan automatically (assuming it was a dumb phone). At this point you have the original phone back on the 'donor' line, the upgrade date for that line is now pushed back 2 years, and you have a new smartphone that isn't attached to any line.
5) On your unlimited line, change the device (not upgrade) to the new phone. Your unlimited line will now have the new phone activated on it. The contract (upgrade) date will not have changed because you didn't actually upgrade that line. And you will still have your unlimited data.
You can do this online yourself if you want, or a store can do it for you. I did it at a local store and had no issues. I should mention that if you go to the store, it must be an official Verizon store, not one of the resellers (WirelessZone, etc). If you upgrade the 'donor' line at a non-corporate Verizon store, when you move the old dumb phone back you will get hit with a cancellation fee. Usually most of these resellers require you to keep the newly added data plan for 6 months so it messes everything up. Amazon and other places have the phone for less than Verizon on sale, but if you are trying to do the extra line method you need to get it direct from Verizon, either online or corporate store.