It goes without saying, but make sure everything is CLEAN. Even the smallest amount of residue can be very attractive to pests.
For plastic furniture, use warm water with mild soap or disinfecting wipes.
For wood, it's best to use specialized wood cleaners, especially if you have older furniture pieces.
For fabric furnioture like couches or seat cushions, use fabric or upholstery cleaner.
For furniture with metal pieces, polish with a polishing cloth and metal cleaner to delay tarnish build up.
FInally, make sure everything is DRY before storing. Vacuum seats or take it apart to make sure there's no crumbs or morsels to draw unwanted attention to your furniture.
Don't CRAM your items and arrange pieces carefully.
Plan ahead for items or pieces that you may need to get out of storage sooner rather than later, and keep them towards the front.
Store heavier items on the bottom and work your way up.
Do not overdo it by putting too much stuff in your unit or stacking on top of unstable items - it is a sure fire way to break something valuable to you, or worse, have it all topple over.
Make sure there is always an aisle to access the back and sides of your unit and leave adequate space between heavy items.
Cramming more and more things in your unit can cause damage to your belongings and if you really need to cram, you probably need a bigger unit.
PRO TIP: When arranging your stuff, start with the largest pieces and work in small pieces at the end.