If these abilities are on the Pokemon in front of your party it'll activate:
Static: See Electric Type Pokemon more often
Hustle/Vital Spirit: High level Wild Pokemon are more common
Intimidate/Keen Eye: Low Leveled Pokemon are less commin
Pressure/Arena Trap: Wild Pokemon more common
EDIT: Synchronize and Compound Eyes are probably the most useful.
Synchronize for natures, Compound Eyes for held items.
Makes it really easy to farm Heart Scales off of Wild Luvdisc (with Old Rod) if you have a Vivillion@Compound Eyes with Thief.
http://www.serebii.net/emerald/ability.shtml More here