How should I store my nibs for long periods?
There are a couple of things you need to consider before we look at storage options. 

Ensure your nibs are clean

When you are done using your nibs you should ensure that you medium is removed and clean before you store it. Most people suggest just using room temperature water or more specifically distilled water (that would have less mineral build up that would being stuck inside the workings of the pen). Ink is obviously easier to clean before it has set.

More on this from Levenger.com

Rust and corrosion

Remember that rust begets rust. You want to be sure that the nibs / cartridge nibs are free of rust before you go to store them. This is especially true if you intend to store your nibs together in close proximity. Enough rust will render the nib useless.

Baking Soda Paste 

For simple surface rust you can make a paste from baking soda and water and use a toothbrush to gently work the surface to remove the rust. Again, using water to clean the surface one done. 

There are more tactics to removing rust, like vinegar or a molasses bath, but given that there are hard to restore if the rust is bad you should only have to worry about simple surface rust. 

Storage

One area where people seem a little divided is on storage of nibs. Particularity when it comes to storing nibs together where they can touch or completely segregated from each other. 

Storage options are numerous. Some styles worth mentioning


Simple tins cans (think Altoids)
Small plastic containers. These are favoured as you can store the nibs vertically. This is done, in theory, to prevent ink build up on the split and tip. 
Tackle box / craft box. Any partitioned storage box really. Multiple nibs that are related to each other are stored together. Since the partitions are configurable or at least different sizes your cartridges would fit in here as well as your pens. 
Cardboard. If you like keeping the nibs separate then this would be a preferred method as each nib would get its own slot. As long as you can keep the board itself in a safe location this also prevents the nibs from moving around which could damage the tips.


Other storage considerations


Cut up small straw lengths that you can either feed down the split (If you have to force it don't do it.) or place on the tip for additional protection. 
Silica gel packets: You save these right? If humidity is an issue for your storage then consider sticking in one or two of these in with your nibs. It will help keep a moisture, and therefore rust, free environment.
If your are fine with storing nibs together make sure at least they cannot come into contact with anything heavier or harder than themselves.