I showed up to the shop, and was given a piece of paper to fill out with maybe 5 questions, and an availability sheet. Sat down with three other girls, and was asked questions by the manager. Then we split up and were briefly taught how to demo a product, and then had to demo it ourselves. Then we had a one on one sit down with the manager, and could just walk around the stuff while waiting for the other girls to finish. Once you were done with your interview you could leave though, even if the other girls weren't done.
My suggestions to get hired would be to be *very* outgoing. As long as you can talk to random strangers, you'll be fine. Be happy and positive. More then anything though just let your personality shine through. If you're trying to fake it, it'll be obvious. You don't have to go overboard to seem fun and likeable. Also, when in the group definitely find a good balance of putting yourself out there, but not looking like you're trying to overshadow anyone else. Help make the other girls look good, since Lush looks for team players.
If you like the company for the cosmetics portion of that, let them know that, but also try to highlight the ethical side of the company as well. If you can even back it up with some of what you do (for example, I'm vegan, so I mentioned that was why I loved the ethics of the company), then you're gold. Talk about your values and how the company fits with that, so it doesn't seem you like you're just spouting off random company facts.
Also, don't worry too much about the demoing part of it. You don't need to know every product before you go there (In fact, I knew none of them, I had just heard of Lush a few weeks before when I applied). There're looking for raw talent, not refined skills. Be confident, and show that you're good at retaining some knowledge (for example, if they list 3 ingredients in the product they're demoing, try to remember at least 2). If you don't remember everything, that's okay. Don't get flustered. There will be times on the shop floor in the beginning where you may be talking about a product and you don't know any of the ingredients. They want to see that you'll remain calm, and be able to work around that.
Also, availability plays a huge key in this. If you're available three times a week you may have a harder time get hired. It's okay to not be available a day or two, and you don't have to be available all day for the days you can work, just try to make it so you can work a few days a week if they need it. Weekends are definitely a huge part of this. If you can't work Fri-Sun at all, you may not get hired, unless they're looking for someone who can specifically work during the weekdays. The weekends though are the time (my shop at least) need the most people. If you need Sundays off, or maybe Saturday, that's fine, but don't block out your whole weekend if you can avoid it.
Be confident, look prepared (but not too fakely prepared), and have fun! That's all there is too it, and then the rest it just up to what the shop needs and who else is applying.