Density is king in a warehouse or distribution facility. The less air you have, the lower your annual cost to store product. So, what are some ways you increase density within your warehouse's racking system?
Hi-Density Mobile Shelving Systems
One option is to utilize a mobile shelving system to store your smaller parts. Systems such as the one below allow you to store up to 2x as much product in a particular footprint.
But, they are really only designed to hold your smaller parts.
Carton Flow Rack
So, another popular option is carton flow rack.
Carton flow rack comes in all sorts of shapes and sizes. It can be retrofit in to your existing rack system or it can be built as a standalone unit. The most dense use for carton flow rack is to combine it with a conveyor in the form of a pick module.
A pick module allows workers to pick and pack from cartons installed in pallet rack. Once picked, they can send the order down a conveyor system to be loaded on to trucks.
In this pick module by SpaceRak / Heartland Steel Products, you can see how the carton flow beds are being installed in the lower half of each pallet rack upright. This lets you store pallets in the higher sections of rack, while allowing workers to pick product from the slots that are closer to eye level.
Pick modules are some of the most dense structures you utilize to maximize space within your warehouse racking system.
Pallet Flow Rack
A pallet flow system integrates selective pallet rack with carton flow to develop a dense solution for storing a lot of similar pallets. Pallets sit on rollers that were installed inside the pallet rack. So, when loading a new pallet, you merely push the current pallet back. When removing a pallet, the next pallet in line slides forward. 
(SpaceRak pallet flow system)
Push Back Pallet Rack System
Basic, selective pallet rack is great for its versatility but it's not very dense. A push-back rack or pallet flow system is much more dense.
(pushback pallet rack)
As you can see in the picture above, pushback pallet rack utilizes a series of carts. You load the pallet on the cart and as each successive pallet is loaded, the previous cart moves back. Similarly, when a pallet is unloaded, the next cart slides forward. It's gravity fed, so there isn't any need to worry about electrical systems.
Drive-In Pallet Rack
Drive-in pallet rack is a good choice have a lot of similar pallets that you are looking to store in a last-in, first-out fashion. The difference between drive-in pallet rack and pallet-flow rack is that drive-in doesn't "flow" forward when you remove the previous pallet.
Instead, a forklift operator drives-in to the rack to retrieve the next pallet.
You typically see drive-in pallet rack in industries that experience sudden influxes of inventory that quickly moves-in and then moves-out.
If you are looking to utilize first-in, first-out accounting in your warehouse rack system, then you can utilize drive-through pallet rack. It's similar to drive-in, but allows you to load pallets from one side and remove them from another.
By utilizing both sides, you sacrifice some density, but it might make your accounting a bit easier.
(drive-in pallet rack)
Conclusion
There are a lot of options available to you if you are looking to remove air and add density to your current warehouse rack system. In order of most dense to lease, they are: pick module, mobile shelving, carton flow, pallet flow, drive-in pallet rack, push-back pallet rack and, finally - drive through pallet rack.