- Organise and neaten: make sure the fashion closet is neat so that when editors/stylists/etc come in they can immediately see what they need, and also so you can easily find items to return to designers/PR.
- Participate in photoshoots: standing around making sure that everything is running smoothly, from doing errands (running off to buy something that's needed asap that they forgot to bring, like maybe white underwear, buying lunch, coffee, etc), make sure the rack of clothes is in order of looks, steam/iron clothes or the fabric parts of shoes/other accessories, etc
- Represent them at events: if they have an event they'll want you to help out, sort of like PR-ing, eg maybe helping out with the list at the front door, etc etc. That's what I'm assuming though, I haven't done this for a magazine, only at a PR agency.
- Search for pictures: sometimes you may be told to look for a particular sort of look from a season and show it to them, or you need to find a runway picture of a certain accessory/item of clothing/on a celebrity/etc etc
I can't really think of anything you can do to really prepare yourself except just to be on your toes and try to anticipate what they want before they say it, listen to their directions carefully, be diligent, etc. It's not rocket science

you really have to make an effort to be a bad intern!
I think i's fine to ask for a week long break, I mean we are unpaid... but not too often an within reason of course!