The best way to ensure you hit with you power attack+charge is the tactical feat Shock Trooper from Complete Warrior. The Heedless Charge section says that when you charge and use at least 5 points of Power Attack, you can subtract that number from your AC instead of your attack roll.
Leaves you vulnerable for a round, but totally worth it imo. In addition, you may want to look into the feat Leap Attack from Complete Adventurer that lets you make a jump check to deal 3x what you subtract from Power Attack when you charge as opposed to 2x. Couple this with the Skill Trick Extreme Leap from Complete Scoundrel that lets you use a swift action once per encounter to move 10 ft. if you have jumped at least 10 ft. this round. Leaping Charge, hit, then use Extreme Leap to move straight back 10 ft. and do it again next round as long as you have a good tank that can keep the mob in position. You take an attack of opportunity, but totally worth it to hammer in a monster's brains for 2 rounds in a row.
Leaves you vulnerable for a round, but totally worth it imo. In addition, you may want to look into the feat Leap Attack from Complete Adventurer that lets you make a jump check to deal 3x what you subtract from Power Attack when you charge as opposed to 2x. Couple this with the Skill Trick Extreme Leap from Complete Scoundrel that lets you use a swift action once per encounter to move 10 ft. if you have jumped at least 10 ft. this round. Leaping Charge, hit, then use Extreme Leap to move straight back 10 ft. and do it again next round as long as you have a good tank that can keep the mob in position. You take an attack of opportunity, but totally worth it to hammer in a monster's brains for 2 rounds in a row.