I imagine it's hard to focus manually on someone when you're trying to get a shallow depth of field effect. I read that you're supposed to focus on the eyes of a person, but how can I tell if someone's eye is focused through the tiny viewfinder? I won't be able to try a DSLR for awhile, so if anyone could give me some tips, that'd be really helpful, thanks!
if you can't tell through a viewfinder, you certainly won't be able to tell on little screen. again, your eyes outresolve even a high-res LCD screen, which the ones on your camera aren't.
back to the definition of an SLR: single-lens reflex. this means you look through a mirror and see exactly what the lens sees. if something is in focus (and the diopter is set correctly), you will see it in focus. if something is veiled by flare, you'll see that it is. it's exactly the same as looking at the LCD screen on a point-&-shoot, since the screen is displaying whatever the sensor sees, but your eyes will be more accurate...and won't get washed out by sunlight.
the sharpness falloff in the viewfinder is also influenced by the focusing screen inside your camera, but that's only a concern if you manual focus often with a fast lens and no AF-confirmation (or you don't trust AF confirm), which you probably won't.
and again, it is impossible to use Live View while holding a camera properly.