Yarr Harr Fiddle de dee,
Being a Dragon is alright with me!
Do what you want 'cause a Dragon is free
You are a Dragon!~

Your most obvious horde (The one you sleep on) is actually a Permanent Image set on top of say, a comfy dragon-sized mattress. This mattress is trapped with persisted Lesser Vigor.
If your dragon wants some extra AC, pick up a Monk's Belt. They're cheap.
the feat Lingering Breath combined with Maximize breath weapon can make things very uncomfortable for anyone not immune to your element. What colour is this dragon? if it's one with the shapechange or alter self abilities, pretending to be another colour of dragon to fool adventurers into taking the wrong precautions will help.

Also, secret backdoor tunnel that is actually a trap. Or rather, a series of traps. First trap: Obscuring Mist (Triggered when any creature opens the secret door) Second trap: Teleport (Triggered when any [your size category] creature moves 20' into the tunnel, to a preset destination [propably also where your horde really is. No dragon should be far from it ;)])
Third trap: Stinking cloud (Triggered when any creature smaller than your size category moves 20' into the tunnel)
Fourth Trap: Ghost sound (Of a dragon running further down the tunnel)(Triggered when a Teleport is activated within the tunnel)
and finally, a delay-trigger autohit mechanical trap for tons of damage, plus poison (Think dropping poisoned ceiling spikes) with a mechanical reset, triggered when a creature steps on a pressure plate. 5 round delay should do it.

Here's how it works: If you are taken under half HP, run for your secret tunnel. Obscuring Mist happens. Intentionally fail your reflex save and trigger the pressure plate while moving forward. You end your turn teleported to your secret location, propably a sealed vault underground which has another teleport trap geared to teleport you back to your cave somewhere inside it. Lesser Vigor means you can just hang out there and regain HP, while back in your lair, the adventurers hear the dragon running further down the tunnel. Anyone who chases you will set off the Stinking Cloud, which will hopefully keep the reckless adventurers occupied in the trap area until the ceiling spikes come down for massive damage.

This is actually fairly inexpensive, the most expensive part being the Teleport trap. The downside is that it only works on foolhardy adventurers who would chase after a retreating dragon in his own lair. The upside is that this accounts for some 90% of adventurers.