There are not many useable ways outside of rogue-like dips.
Craven, which increases precision damage, requires you to have SA in the first place.
Assassin's Stance (Shadow Hand 3) allows you to gain SA without class levels, and you could get there through a Cleric dip with the War Domain if you already have Weapon Focus, Martial Study and a high enough initiator level, but... That's awkward and probably not worth the effort.