Getting back to the big picture for a moment, there's a few tactics that could work quite well.
First, here's one tactic that could seriously root an invading force if the goblin warren is a single cave system, with one main entrance: The moment you're invaded, get everyone out through side entrances and cave them in behind you. Then set up a barricade at the front door. Suddenly you're besieging the people that came to root you out. This works better if you've got food stores in packs so you can haul them out with you, maybe some tents, or better still some dead animals you can leave in the cave to infect your foes (the dead animals can be eaten if you're not invaded; just make sure you have a steady stream of corpses passing through.)
- This technique gets around the flaw that an enlosed cave system is easily 'smoked out' or starved.
- I used this tactic once with a warband of about thirty goblins. When the heroes got damaged and decided to leg it back to town for a rest/heal, they found themselves closed in against overwhelming odds. And there was only one exit. The goblins started piling up rocks.
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Second, the Underdark. Essentially a big cavern system without end. If you map out the area around you, you'll have an immediate advantage if you can keep your population mobile, and keep away from invaders through use of skilled scouts. Humans invade, find nobody, and take off. The next day, the raiders are back. This one even works against powerful invaders who scry out an area and teleport in after days of prep; if you scatter and meet up five miles away in a place no wizard has ever seen, you will at least survive.
- Again, no smoking out. Starvation is a problem unless the goblins can hunt underground. Mushrooms aren't exciting, but they're prevalent. Consider food sources.
Third, alliances. Goblins are weaker than anybody else except maybe kobolds in the humanoid stakes; how have they survived so long? By making friends, I'd guess.
- Intermarriages between tribes could easily create bonds that moderately concerned goblins would act on. So there's assistance from local goblins.
- Better than enslaving a bunch of kobolds, keep their communities on a short leash as buffer states. Raid them occasionally. If one of them angers you, massacre them all, roll their skinned bodies in salt, nail them up as an example etc. The key here is keeping several weak communities around - goblin overseers (of high level and discipline) could take the role of ambassadors and beat the crap out of anyone who starts organising the kobolds to resist their overlords. The kobolds absorb invaders, give advance warning, and trap the heck out of their environs. They're slaves with illusions of grandeur, which would satisfy a lazy goblin.
- If you don't have anywhere else to turn, become someone else's vassal. Frankly, this is where most goblins find themselves. A lot of very powerful creatures live underground; a band of men-at-arms can hew down goblins, but an inquisition of illithids (a mere handful) will psi-blast the lot of them and eat their brains before you can say "Jack Splat". Or make a deal with a dragon and hang out in its cavern a lot. Or make friends with an umber hulk cluster, which can do your tunnelling for you.
Paying these powerful allies can be difficult, but remember: Goblins breed like rabbits. It's the only way they could have survived the rise of humans, considering the average goblin is fairly sucky. In fact, I'd bet most goblins aren't even adults yet, hence the slovenly behaviour and laziness you'd expect from a teenager. So feed your overflow population to your overlords. Sure, goblin brain isn't as tasty as that of an elf to an illithid. But more potatoes are eaten than caviar in this world.
Fourth, I like the idea of a preemptive antistrike technique: Why do the humans want to attack you? Because you've burned their crops and stolen their women? Why bother? Much the same way as you could keep kobold tribes around your subterranean lair, keep some human communities that you actively protect from other threats (or at least pretend to - "Gee, thanks, Mister Gruk-Thug, that's the third dragon you've driven off this week! How can we ever thank you?") and demand some small recompense from. The villagers will actually fend off goblin-slaying adventurers rather than be left vulnerable. (This works well in remote wildernesses where goblins tend to live, as the villages will be under constant threat of wild beast attack, and before someone says wolves don't attack humans, it's just that the bodies were never found.)
- This may cause problems with the general goblin populace, who see the humans as either enemies to be defeated or slaves to be cowed. You'd have to train a number of leaders to rein in their grunts, and keep them off the humans - for the most part; it's a case of balancing the human's need for protection with their revulsion of evil overlords. Pushing humans over in the street is not advisable, and can cause inconvenient heroism. It's best to wait until you can catch them in the woods and blame it on a bear when the other humans come looking.
- You are still free to raid more distant villages; in fact, your protectorates can serve as base camps. This works best if you're near a border and can raid foreigners. You might even be honoured by the king of the humans you protect for 'services for the Crown' (beating on his enemies).
- It's always best to show off how mighty you are. Frequently ride through their streets on your worgs. The fact that many worgs speak Common should impress the villagers and keep them cowed. Oh, and a worg makes an excellent spy for just this reason. Hide in the bushes and watch the humans go about their business. If it's spotted, the goblins show up and chase off the 'wolf'.
- All in all, treat humans as cattle: keep them safe from predators, and live off what they give you. Admittedly it involves a little work, but see my next point.
Fifth, it's always best to keep your populace well-trained. If you're going to survive by raiding, raid often to gain experience, and weed out the slackers in your ranks. Trim your population down to the veterans. A few good soldiers are an invaluable commodity, primarily as leaders to keep the grunts in line. (And given goblin's fecundity, you'll have a lot of grunts fairly quickly.) I realise goblins aren't normally like this, but a clever leader should be able to instill some discipline and motivation into their minions, and the minions can beat on anyone who disagrees.
- Depending on how effective this is, a few generations of selective breeding could produce stronger, more intelligent goblins that can work together for distant goals, rather than lie around all day picking on the little guy. Perhaps this is how hobgoblins got started.
Well, there's some strategies that might work. Imagine the situation: PCs ride into town looking for goblin raiders, but are greeted coldly and told to get out if they reveal their mission. When they find the goblin lair, it's been trapped by kobolds, and if they wipe out an entire tribe of the little pests, they find a hastily abandoned central complex. As they go to leave, puzzled by the lack of goblins, they discover that they've been walled in... and the dead kobolds are starting to smell.
Or worse - the goblins pull further back into the caves, fighting a rear-guard action on terrain they know, led by high-level fighters. When the PCs think they have their prey cornered, they charge ahead - and find themselves staring at a trio of umber hulks. If they can avoid killing
themselves in the sight of
confuse eyes, they've still got a heck of a fight on their hands.
And if they finally beat the goblins, despite all odds, they come out and find that they're wanted by royal edict in this kingdom. Suddenly killing goblins isn't the fun trip it was supposed to be.
