I'm going to try to take as little as possible. Basic toiletries, a couple pairs of clothes, a good supply of my usual vitamins/supplements, a few books, and my laptop (probably). There's no reason to bring too many things. As far as formality goes, it depends on the country, but keep in mind that most countries require more formal dress in educational settings than you might be used to. I know this is true in much of SE Asia where students often wear uniforms. I expect to be wearing a nice shirt, slacks, and a tie every day as an ETA in Thailand. However, if you don't have all the formal clothes yet, I would NOT recommend running out and buying them before you leave--you should be able to get very nice clothes in your host country for a fraction of the price you'd pay in the States (for instance, I've been offered tailored suits in Bangkok for 60-80US$, with a few extra shirts thrown in). For women in particular, this will also ensure that dress clothes fit with local norms as clothes showing shoulders, chest, legs, etc. may or may not be acceptable, depending on the culture.