Let's set the scene: You're looking for a new apartment or house. You walk into a cavernous and empty room with a hard wood floor. You're looking around and seeing the potential. The couch could go here. The TV would have glare if we put it on that wall. You open your mouth to speak your concerns to the landlord or realtor, and you notice it...
The house feels empty because it sounds empty. There's a coziness to a lived-in place primarily because sound dies on the things you surround yourself with. Your bed, the carpet, your keepsakes... all of them stop sound waves from bouncing around all willy nilly. These things don't just fill the space in a room, they bring the walls in around you and make you feel safe.
An echo-y room reminds us of gym class or the warehouse where we will eventually meet our bloody end. It's distracting.
That's why, in podcasting, it's important to get your sound treatment shit right.
There are technicians with degrees in the physics of sound who can explain this a lot better than I can, but the basic principles are simple. Sound waves bounce off of flat hard surfaces. When these waves bounce back to you, they create an echo. Large flat surfaces that are parallel to each other create particularly nasty echoes. You want to eliminate these echoes.
Think about the room where you record. If it's empty, you have three sets of parallel flat surfaces working against you. The wall in front of you and the wall behind you. The walls to your left and right. The ceiling and the floor. Everything you put against those walls to interfere with that bouncing will deaden your room and improve the quality of your audio.
It's difficult to search for soundproofing or sound deadening advice online because most people are concerned with keeping sound in or out of a room. That's a different problem, and one that I'm not great at solving. However, I can give three tips on how to reduce echo in a room.
Pick your room. Bigger rooms tend to be more echo-y because they're less likely to be full. My "studio" is the second bedroom in my two bedroom apartment. I keep a bed in here in case I have guests over. I've also got a bookshelf in here, and one wall is a big closet where I hang some clothes (and I keep the door open while recording). Soft pieces of furniture, or furniture with a non-uniform surface deaden sound, so go nuts decorating.
If you can't record in a small room, try to divide your recording area off into a smaller portion of the room. Gary did this by putting a book shelf behind his desk chair and draping a blanket over the back of it. You would have no idea that his recording room is as big as it is from the way his audio sounds. It also helps that his room is carpeted, which brings us to...
Get yourself some carpet. Bare hardwood floors are trendy and nice, but they bounce sound around like a motherfucker. Even a shaggy 5' by 7' area rug in a 10' by 10' room will make a huge difference. That's my situation, and there's very little exposed hardwood in my studio. There's likely a discount retailer near you that sells moderately priced rugs, and they will have a huge effect on your acoustics. Also, carpet feels nice against your bare feet and vacuuming is fun.
Buy your way into soundproof heaven. If you're willing to spend some money on sound treatment, it will go a long way. If you look at professional studios, they always have some kind of foam paneling on the walls. This stuff is specially designed to reduce echo, and it's not terribly expensive.
I have twelve 24" by 24" panels hung up all throughout my room, mostly on the walls in front of me and behind me. I bought them from The Foam Factory for about $75, shipped. It comes in a neat little box all shrink-wrapped. It's fun to watch this stuff expand after you cut it open.
Pro-tip: Never, ever go to Guitar Center unless you absolutely need something right away. They will gouge you on every single item you buy. Compare my $75 foam to the Guitar Center equivalent, which runs a horrifying $229. That money is better-spent elsewhere.
Most online retailers sell multiple "patterns" of foam, and they make a case for why a certain cut will be better-suited for particular uses. I'm sure there is some kind of basis for this, but it also sounds like hocus pocus. I chose wedge because it's a simple pattern and also less costly than other cuts. I won't judge you if you go for a fancy, more aesthetically busy pattern. It's fashion.
There are multiple ways to hang this foam. Some places sell adhesive spray, but if you rent your place, you might worry about damaging your walls and losing your deposit. Depending on what material your walls are made of, you have some options.
If you have relatively soft drywall or sheetrock, you can use t-pins. A handfull of these will hold up each panel, and will only leave small pin-holes in your walls, which you can easily fill with Spackle or toothpaste when you move out.
If you have hard plaster like I have, t-pins won't work. Never fear, because you can use adhesive foam mounting tape or strips. I can't currently find the non-permanent versions of the Scotch mounting tape online, but they exist. Even small strips can hold up a large amount of foam, so don't go overboard. If you're worried about using any kind of adhesive on your walls, find an out-of-the-way patch to test on. If it pulls up any paint, try something different. When you're ready to take your foam down, carefully peel the foam from the tape, and then carefully peel the tape from the wall. This will reduce the risk of damage to the paint job.
When you hang your sound treatment, you don't need to create a continuous surface of edge-to-edge foam. That can get expensive in a hurry. You can leave strips of wall between your panels. This contributes to making the shape of the surface more complex, since the edges of the panels are exposed to create more surface area. I wouldn't hang the panels more than 1' apart though.
If you don't want to spend money on foam, you can hang good old fashioned blankets and towels from your walls. I personally wouldn't do this because it doesn't look good, but it's an option that works similarly well if you don't mind the aesthetic hit.
Hopefully those are some practical bits of advice if you're looking to kill the echo in your recording. If you have any control over the space you're in, even the smallest steps will make a noticeable difference.