Like Nail said, archers are plenty good enough already.
So, let's look at a good archer and see where the damage comes from:
12th level fighter archer--damage 1d8+13 (+12 beyond 30')+1d6 (fire) +1d6 (electricity)
So: 1d8 (base bow damage) +5 (strength--starting 16, includes +4 belt), +3 enhancement (Greater Magic Weapon from party wizard or cleric), +4 greater weapon specialization, +1 point blank shot, +1d6 fire from Flame Arrow (party wizard), +1d6 electricity +1 mighty [+5] composite longbow of shock.
If you're not a fighter, you can drop the greater weapon specialization off of that. Now you're at 1d8+9 +2d6. Still quite respectable. A skeleton's DR 5 won't shut you down (though an elemental's DR 10/- will--elementals and other DR /- creatures are the bane of archers (and two weapon fighters).
Let's pretend that you're not that strong though. You started with a 12 strength and didn't buy any items to increase it. Now you're down to 1d8+5 +2d6. Now, you're really relying upon the elemental damage and, if it doesn't apply, you'll have trouble with skeletons of any stripe and elementals are out of bounds completely.
Let's further pretend that you don't get any buffs from your allies and now we're down to 1d8+3+1d6. Not impressive at all.
So, what's the lesson:
1. Weapon specialization and greater weapon specialization make a HUGE difference. If you want to be an archer, fighter levels are the way to maximize your damage.
2. Strength is not optional for an archer. If you need a wimpy [+x=<3] bow rather than a mighty one, then you're at almost as much as a disadvantage as a melee fighter who skimped on strength.
3. As with all character, buffing from your party will make a big difference in your performance.
Now, what if you don't have any of those? How do you deal with it?
First things first: if you have no strength, no weapon specialization, and your party doesn't buff you, then you're not doing very well and have little claim to be an archer. A rogue could fill all of the same roles, scout, find traps, etc. and SNEAK ATTACK if properly buffed (Gr. Invis, etc). Creating a gimped character and wondering how to do damage is like buying a motor scooter and wondering why it has trouble accellerating to freeway speed--it's an utterly predictable and only partially solvable problem.
2. Make use of the abilities you do have. If the character's claim to be an archer has any validity and he doesn't have weapon specialization, then he ought to have Divine Might or Favored Enemy. Choose and use those well. A 12th level ranger could well have +6 damage against elementals, +2 damage against undead, and +2 against magical beasts. That's almost enough to bump his damage up one category--two in the case of the +6 favored enemy.
3. When it's DR that is the problem, take steps to overcome it. Generally, archers have less problem overcoming DR than melee fighters because of the ease of aquiring multiple "and" weapons. Good and cold iron (high level demons)? I fire cold iron arrows from my holy bow. Silver and magic (vampires)? I fire silver arrows from my magic holy bow. Silver and good (high level devils)? I fire silver arrows from my holy bow. Silver (Lycanthropes)? I fire silver arrows from my holy bow. Golems? I fire adamantine arrows from my holy bow. I'm sure you've noticed the common thread: the archer never has to change weapons--he just has to pick the right arrows--and unlike the right magical or masterwork melee weapon, the right arrows are CHEAP. That works against most kinds of DR you encounter.
What about the other DRs though?
1. Clay golems have DR adamantine and bludgeoning. The holy bow doesn't do that.
Answer: A. Every character has a weakness. If your arrows don't work try something else. Point blank shot, etc apply to tanglefoot bags. They apply to acid flasks. There are still things you can do. Heck, if you're an archer, you probably have a pretty darn good BAB and if you're a fighter, you've got more feats than you need for just archery. Pick up power attack and fight like a man every now and then.
B. Golembane scarabs are cheap and let you ignore ALL golem DR. If you run into golems often, it'll even save you a fair amount of money--adamantine arrows (unlike silver and cold iron arrows) add up pretty quickly.
2. Skeletons and other creatures with DR/bludgeoning.
A. Remember that bit about tanglefoot bags, melee weapons, power attack, and occasionally varying your fighting style.
B. Slings are cheap and will deal a fair amount of damage if you've got a decent strength--even more if the cleric casts Magic Stones for you beforehand.
C. Quickdraw (remember the bit about fighters having plenty of feats) and light hammers. This also gives you a lot more flexibility in using option A.
4. Elementals and creatures with DR /-.
These are the bane of archers--no question about it. Of course, they're also (and even more) the bane of rogues (beat DR 10/- without sneak attack Mr. Weapon Finesse!), two weapon fighters, and sword and shield types.
A. Pick up a melee weapon and fight it out.
B. Keep firing your bow--if you deal damage like the fighter up top, you still beat the DR on every arrow.
C. Get an elemental bane bow for special occasions (or as an enhancement on your main weapon).
D. Favored Enemy: Elementals.