I keep mine indoors on cardboard covered shelves in a cabinet I bought specifically for storing painting, wallpapering, sheet-rocking and other tools & supplies.  

Other paint storage tips

I use a permanent marker (Sharpie) pen to write notes about the paint on the bottom of the can [E.g., Sherwin-Williams, B-24 R-66 (color code), bathroom trim, 12/2007 (date purchased).]  If you won't be able to see the bottom in it's storage location also write the info on the side of the can, or on a label you put on the side.

The key to storage is keeping the air out of the can.  Don't wipe the brush on the lip/rim of the can while painting.  Paint will, get in the grove (called the "chime") and make it difficult to seal the lid properly.  Before closing the can, carefully wipe the 'chime' and the rim of the lid free of paint residue, especially if you didn't follow the suggestion about not wiping the brush on the lip. 

To keep the chime clean, I use a plastic pouring lid/cap that inserts/snaps into the rim of an open gallon paint can.  It keeps the rim clean while I pour the paint into a smaller container for easier handling while painting.  Some paint today comes in cans with a pouring thingy built-in.

Hammering the lid directly to close the can will distort the lid, preventing a good seal.  The common advice is to close the can by putting a block of wood over the lid and tapping with a hammer, all the way around.  I use a large rubber mallet. 

Use a "key" -- a tool specifically made to open the can by hooking under the lid's rolled edge without distorting it.  Most paint stores will give you one.  Prying up the lid with a screwdriver is likely to distort the chime.

Another way to seal out air is to drape a piece of plastic wrap into the can and floating on the top of the paint.  In this case you store it right side up.  I've never tried this, I always just store them upside down, but if this works it has the advantage that when you later open it, if a “skin” has formed on top of the paint you can use a spoon or paint stick to remove the skin (or it may lift off with the plastic).  When you store the can upside down the skin, if any, forms on the underside of the paint.  When you open and use the paint you can’t tell it’s there or get to it, so you'll stir/mix the skin into the paint.  Then the paint will carry pieces of the skin onto your work.

One writer on alt.home.repair advises, “After closing the lid on the paint can, turn it upside down - THEN store it right-side-up.  This coats the inside of the lid-to-can seam thus sealing the contents against drying out.”  I don't know if this works either but if it does it has the advantages with regard to removing the skin.  

When I’m done with a gallon of paint, before I put it into long term storage I use a basting spoon or small ladle to put some in a small bottle/container, and label it (E.g., bathroom trim).  Then if I get a ding or put spackle on a nail hole, I get out the bottle of paint and my trusty Dollar Store set of large artist's brushes and touch up the spot.  (Tip:  I'll usually blend better if you dab instead of stroking the paint on.).  NOTE: Fill the bottles to keep air out.  So far, even filled, these little bottles of paint dry up fairly quickly (2-6 months).  If I find a way to make this touch-up collection last I’ll let you know.

If you store your leftover paint in containers with screw on lids, paint often gets on the lid threads making it difficult to remove the lid.  To prevent this apply a very thin coat of petroleum jelly to the threads inside the lid. This will also keep metal lids from rusting.