Props make a photo booth more fun, and can get people into the photo booth who would normally feel shy about going in. Props help break the ice. Props bring out peoples creativity and make for more exciting photo booth pictures.
Props don't have to be expensive, in fact I hate to spend more than $10 on a prop since they often get damaged or go missing during an event. While they are an added cost to doing photo booth business they are almost as important as owning the photo booth itself.
Here are a few of my recent favorites
The angel halo and devil horn combo. These are very popular and bring out the naughty and nice in your photo booth guests.
The mustache and lips on a stick. Stick props are an easy to use, they don't mess up anyone's hair, and frankly who doesn't look good in a mustache. Get paper ones on amazon.com just search "photo booth props" or for a more durable prop cut them out of black and red stiffened felt that you can purchase at a local craft store and then hot glue them to bamboo BBQ skewers. Popular patterns include mustaches, lips, glasses, old fashion pipe, beards, monocles, and bow ties.
Viking Helmets. You had me at Viking!
Mario and Luigi hats... Easily found on amazon.com. "It's a me... Mario!" (goes well with the mustaches on a stick)
Dollar store plunger! Way more fun than having to use one at home....
Feather Boas are great but they shed and make a mess, and don't last long. My trick is to use wonder scarfs, they photo just like boas and last a long long time. They come in many colors, They cost about the same as a feather boa that won't last 2-3 events, and they don't shed.
Dollar store magnifying glass. Fun in the booth, big eyes, big nose.....
Boxing Gloves. at about $16 on Amazon these are near the top of my recommended spending but they are basically two props and are large enough that they don't often disappear. Many creative poses come from these, and guys love it which is important since women are more easily enticed into going in the photo booth.
And last but not least. My favorite photo booth prop of all time (and also my most stolen... It's crazy how popular this prop is) The Rubber Chicken!