How to avoid being obvious. There are three watchwords here.
Of course this requires planning your room descriptions out ahead of time. My model for this is the 1st Edition DMG.
"DM:'The sacks hold rotten grain, so the cleric will go and help the magic-user as ordered. They find the refuse consists of castings, some husks of small victims of the spider, hide, bones, a small humanoid skull, and 19 silver pieces. Do you now fire the webs overhead?'
LC: 'Examine the skull first. What kind of humanoid was it? Can we tell?'
DM: 'Possibly a goblin. When you are looking at it more closely, you see that there is a small gem inside - a garnet.'"
DM:'First, the others checking the containers find that they held nothing but water, or ore totally empty, and that the wood is rotten to boot. You see a few white, eyeless fish and various stone formations in a pool of water about 4' to 6' deep and about 10' long. That's all. Do you wish to leave the place now?"
LC:'Yes, let's get out of here and go someplace where we can find something interesting.'
OC: 'Wait! If those fish are iust blind cave types, ignore them, but what about the stone formations? Are any of them notable? If SO, I think we should check them out.'
DM:'Okay. The fish are fish, but there is one group of minerals in the deepest part of the pool which appears to resemble a skeleton, but it simply - '
Be Subtle: This means downplay the things you mention. Mention them as if they are unimportant items in the room. There's a yellow cloak, some leather boots and a sword. Is the cloak covered in yellow mold? Do the boots hold a key? Is the sword rusted or magical? If you just described 10 other things in the room will the important thing stand out?
Walls aren't soot covered or covered in blood. Walls are stained, dirty, dark, filthy. The floor isn't covered in pulverized rock, the ground is sandy, dirty, or dusty.
Be Complicated: The way into the secret room, isn't always in this room. Secret doors are not always two way. Your hints, maps, treasure maps and clues can be abstract. Mechanisms can be specific (You must lift up on the east side of the slab, the spider decoration on the iron chair in the corner must be lifted, the right eye of the frog mosaic must be pressed in 3 times.) For the uninitiated, here is Why This Is Not Pixel Bitching.
Be Crafty: This is, know what your players are expecting. Did you trick them with yellow mold once? So they are expecting gold or yellow things to be dangerous? Put a gold or yellow thing on a wall that when poked, triggers a trap. When the poke it with a stick, to test if it's yellow mold, they get hit with the trap! Don't design your encounters in a vacuum. Have a repeating feature in your dungeon, like round stones in the wall. Have one of the stones depress to deactivate a trap, and another that presses in to activate a trap. Then leave a map as a clue, but make the map vague and abstract. Engage your players, make them think!