There are two Accessories I recommend having equipped to make skill grinding go much much faster:

Training Ring II
Equipping this will raise the skill EXP you gain from using skills from 1 to 3. This ring triples EXP gain and will cut down time spent leveling skills significantly.

Equip this on the character who's skills you are wanting to level up.

Energy Orb
Equipping this will let you gain SP from simply running around. This can help save time as its another way to get SP on top of the green crystals. This item will not work if you don't have it equipped on one your main party members (the 3 visible characters you control)