I love barbarians, in fact, that's the class I usually choose to play as. Ever since I read the Conan stories by Robert E Howard, it just dosen't feel like an adventure for me unless one of the party members is a tough-as-nails barbarian warrior.
Here are just some tips I've gathered from playing:
Invest your highest ability score in strength, since Barbarians tend to be melee powerhouses. Dexterity is useful for dodging attacks. Constitution is a must as well, because while barbarians tend to deal a lot of damage in combat, they tend to absorb a lot of damage as well.
Like other people mentioned in this thread, use Power Attack a lot, especially on charges.
Using a nice two-handed weapon will allow you to really dish out the damage in battle. A greataxe or greatsword will do nicely. Another weapon that could work well is the oft-overlooked falchion, which deals good damage and has a high threat range.
Speaking of weapons, try to get your hands on a mighty composite longbow if you can. You never know when you'll be forced to fight enemies that are flying or too far away to engage with melee weapons.
Cleave is a very good feat for a barbarian, and if you know you'll be facing hordes of weak enemies, you might as well take Great Cleave as well. Few things are more satisfying than killing seven or eight enemies in a single round.
One of the major weaknesses of the barbarian class is that they tend to have low AC, since they usually wear light armor to keep their fast movement, and the rage ability reduces AC. Because of this, the dodge and mobility feats are useful for a barbarian. Especially mobility, since barbarians will often draw a lot of attacks of opportunity when they charge into a pack of enemies.
The other major weakness of the barbarian class (and warrior classes in general) is low will saves. So you may want to invest in the Iron Will feat if you can, since it will give you a little better chance at making those will saves.
Whirlwind Attack + Great Cleave is a
very nasty combination, so you may want to invest in it if you can. Unfortunately, meeting the requirements for Whirlwind Attack it requires that you have an intelligence score of at least 13, and intelligence usually isn't a high priority when barbarians are assigning ability points. Another downside of this is that you have to take a lot of feats before this can happen. So the whirlwind + great cleave combo is more plausable for fighters than barbarians.
Multiclassing can give a little spice to your barbarian, and taking a few levels in fighter can give him a little extra edge in combat due to bonus feats. My current barbarian gains his multiclass levels in a 3:2 ratio, so when he eventually hits level 20, he'll be a Barbarian 12/Fighter 8.
A downside to multiclassing as a barbarian is that you might not get some of the cooler skills the barbarian gets later, such as Greater Rage. So you'll have to decide whether multiclassing is what you want to do.
These are just a few tips I've gathered from my study of the barbarian class. I hope I was able to help, and I'll definately be watching this thread to see what other barbarians have to say.
