Never used an Iphone camera, so I can't help with specifics, either. Generally, though, I'd advise using daylight over lamps - you'll get truer colors and more even lighting. Lamps tend to cause glare unless diffused (the same is true of direct sunlight, but it's easier to find reasonably diffuse natural light).
For the white background sheet, try and set the model near one short edge of the piece and prop the back up, leaving a gentle curve behind the model.
Also, no matter what camera you use, the more stable you can keep it, the better. If it has a timer function, line up your shot, prop the camera in place, and use that to ensure your hand won't shake it. Barring that, simply brace your hands on something to avoid blur and focus issues, as much as possible.
Speaking of focus, I'm sure everything is automatic for the camera and that it lacks a macro mode (actually, I'm less sure on the latter part - use it if you've got it!). If you're having trouble focusing clearly, try simply moving back. You can also occasionally "trick" the lens if it insists on focusing somewhere else (nothing like being able to read the homework way at the back of someone's painting desk while the model they're "showing" is a big beige blob).
If the camera has a delay between focusing and taking the shot, toss a hand or something in line with the mini, giving the camera a bigger target. Auto-focus tends to go for the closest crisp image it can get to its current state, so getting it to focus short might help it back up to the plane of your model, not the other side of the room. If not, try taking a dummy shot of something else at the right focal length to have the next try start in the same ballpark.
Finally, preview your pictures before uploading. I know it seems obvious, but somehow unrecognizable images keep creeping on here, often excused by the poster with the word "Iphone" (although by asking, you've already proven yourself to have the right idea

). Playing with lighting, distance, etc. can do a lot for your pictures, even with a less than ideal camera, just as a lazy and inexperienced photographer can pump out crap, even with high-end equipment.