When you are trying to outlast the other competitors you need every little bit of help you can get. Here's my recommendation for equipment.
Stein: Probably the most important tool you can get for training, and honestly, the only real piece of equipment required for steinholding is, not surprisingly, a stein. Sure, you could do some of your training holding similarly weighted dumbbell, but it isn't going to feel the same and it isn't going to prepare you as well for a competition since the grip is way different.
Steins come in many shapes and sizes, but you want a glass 1 Liter stein that weighs about 3.5 pounds. With a liter (~33.9 fluid ounces) of liquid in it, that stein should weigh approximately 5.5 pounds.
In competitions, the stein is likely going to be filled with beer or sometimes water depending on the venue. There isn't honestly a significant difference in the densities of water and beer, so for training purposes you'll want to go with water because once you start training hard you should be spilling all over the place and you don't want your house smelling like the basement of an off-campus fraternity house. Maybe you do, I guess I don't know. You probably don't.
If you are entering a competition and it's at a bar after work or something, take a change of shirt with you. You do NOT want to be wearing a dress shirt for steinholding. It is going to be too tight across your back or chest or shoulders and it will mess up your range of motion. Go for a polo or t-shirt or something that has a little stretch to it. If you want to strike fear into the hearts of your opponents, you should probably be wearing an official USSA t-shirt.
For training, I just recommend whatever type of shirt you would normally wear to workout.
Over a couple of years of training I figured out that the shoes that I train and compete in make a surprising difference in my performance. It might sound ridiculous, but it actually isn't.
Almost any athletic shoe you buy is going to have what is called a "heel to toe drop" or just "drop" value. This is the difference between the thickness of the sole at the heel and the thickness of the sole at the toe. The higher the drop value, the more the shoe will raise your heel up and pitch your posture slightly forward.
I have found this to be a bad thing for steinholding so I recommend a "zero-drop" shoe. This means that the sole is flat, which helps you rest more naturally on your heels when you are holding a stein, creating a more stable base and helping you keep better posture. If you have a pair of zero-drop running shoes available, try it for yourself and you'll probably notice a difference.
I initially got into zero-drop shoes a couple of years ago because I was having shin-splints when I was running in normal traditional running shoes. I switched to zero drop shoes and it made a huge difference for me. Zero drop shoes cause you to change your stride from a rear-foot strike to a mid-foot strike, allowing you to absorb more of the forces via muscular control of joint bending at the ankle and knee, rather than absorbing those forces up through your calcaneous directly into your tibia. Or so my doctor tells me.
I happened to wear those shoes a couple of times while doing practice holds and I found that it was way more comfortable for me. So double win.
I don't currently have any endorsement deals (SECRET HINT: if you pay me or give me free stuff I'll recommend just about anything), so I don't necessarily recommend this brand over other similar brands, but these are the shoes that I trained in and wore during the 2015 season and championship, so I know they're good shoes.
If you don't run, you can probably find way cheaper options than some of the pricey zero drop running shoes out there. If you do any power lifting, you may already have a pair of flat soled shoes like Converse All-Stars for squatting or deadlifting for pretty much the same reason: they don't pitch you forward and compromise your form. To me these aren't as comfortable for long-term wear because they don't offer much in the way of padding, but for training or competitions they will do the job and they are a lot more affordable.
You probably won't be allowed to compete if you aren't wearing pants. I recommend something intimidating, like corduroys. I think they look great and I would wear them all the time but for some reason I get really overheated when I wear corduroys. If you sweat like me, you'll want to go with shorts.