Use both hands

D&D 3.5 rules:
The longsword is one of the most commonly selected weapons. If you use both hands to wield it, you can apply 1 and 1/2 times your strength bonus to damage. But two-handed use of a one-handed weapon has even more advantages if your character has the Power Attack feat. Power Attack allows you to add twice the amount you subtract from your die roll to your damage roll. Thus, a character with a strength of 18 (+4) and a Base Attack Bonus of (+5) and with the Power Attack feat could do either of the following:

a) wield the weapon in one-hand and hit for d8+4 damage, or

b) wield the weapon in two-hands, use the maximum Power Attack (+5) and hit for d8 +16 points of damage.

Breaking this down, the use of two hands increases your damage from Strength bonus of +4 to 1.5 * your Strength bonus (+6). By using Power Attack and subtracting 5 from your attack roll you can add 5 to damage; but when using the longsword with two hands you double the power attack damage bonus (+10).