Taking photos in Sims 3 is basically a hobby of mine. Used to take tons of photos in Sims 2...too. Hope I don't sound too much like an echo, but here are some tips I've picked up over the years:
-First, walls up. I suppose this is more of a roleplay thing, like do you want your photos to be obviously from a game or not? I'm obsessed with making sure that walls and ceilings are up. It's not always easy to take photos with walls up, though. Certain houses are really bad for it - houses that are too small will make it impossible for you to see your Sim beyond the wall, and houses with large rooms will expose the ceiling. If you build your own homes, just try to be aware of stuff like that. If you're like me, like to decorate but not build, then you'll learn quickly what houses are good for photos and what are living nightmares. Unfortunately, the only way to really learn is trial and error and becoming familiar with the houses. One of my favorite houses, Loft Aloft in Twinbrook, is a terror to take photos with the walls up.
-As others have said, zoom in. But don't just try to zoom in - try to angle the Sims in the center as much as possible. And try to get their faces as much as possible. Sims make amazing expressions, which you will miss if you take mostly profile shots.
-Take a lot of different shots from a lot of different angles. Photos look different in the game than they do later in your screenshots folder. Some of my favorite pictures (when I took them, at least) have turned out to be duds, while pictures that I thought wouldn't work turned out great. Even the highest-performing Sims games tend to have some clothing glitches, such as hair poking through a shirt or something, and you're more likely to get a shot without that effect if you take a bunch as opposed to just one or two. I delete about 3/4s of the pictures I take.
-Pause frequently. 'Nuff said.
Despite being so into Sims photos, I didn't know about the Tab key...
Spent like a million years using edge scroll. Will have to start using it from now on!