Hand selection is probably the single most important factor in Omaha H/L winning formula.  You should choose hands that have the potential to have the high and the low.  For example, the best hand is A-A-2-3, because of the potential to form sets if an A flops or to form the nut low if a 4-, 3-, 2, or A- low flops.  But how to select a hand when you are just playing run-of-the-mill cards, like A-K-J-3?  All of these sub-standard hands should be discarded.  You should only play the hands that have potential to be the nuts.  With A-K-J-3, even if it is double-suited, what can you hope for?  A low might give you the second-best low in a multi-way pot and end up costing you a HUGE amount of poker chips and consequently, lots of money. 

Thus, it is best to stay away from most of these hands.  You should confine you hand selection mostly to low cards, like A, 2, 3, 4 etc.  In fact, one Omaha H/L pundit, John "Miami" Cernuto, has opined that the secret to successful Omaha H/L selection is "three cards to a wheel."  This underscored the importance of chasing the low side of the pot, rather than pinning your hopes on some mirage of flopping the best high hand with A-K -10 or some such combination of cards.  Thus, we arrive at the crystal of wisdom: "Never leave home without an Ace."  An ace has great two-way potential as the top high card and as the winning wheel card if a 2 flops or some other wheel card.  Even better is an ace paired with a 2 or a 3, because then you have a chance at making the nut low if for example, a 2 flops and you have A -3.