Tip #1: When doing themes, it's best to plan it ahead of time. It's easier when you have an idea of what to do before you go into the game. If it's a little difficult to think of anything: find inspiration in other photoshoots, listen to music, or ask others for help. When I was doing the 'Bette Davis Eyes' photoshoot for Top Sim Model Cycle 2 (still my favorite photoshoot), I listened to that song constantly before and during the photoshoot. When you're doing the 'omg look at my new sim!' casual photoshoot, you don't have to plan anything out :P
Tip #2: Tab is your friend! You will get clearer pictures this way. You use the Z key to zoom in, the X to move out, the Q to move up, and the E to move down (I'll double check the keys later, I didn't write them down). Use the Print Screen key when you take pictures also. Your pics will have more detail if you use that instead of the Maxis camera.
Tip #2a: I use GunMod's Camera Mode (which has more of a range than default cameras), and you can find the link here: http://www.modthesims2.com/showthread.php?t=97642
Tip #3: Just experiment with different angles, and be sure to take a lot of pics. You can always delete what you don't like.
Part I: Lighting
In my game I use GunMod's Radiance Lighting from Mod The Sims 2 (link is here so it's more dramatic than default lighting.
Tip #4: When using GunMod's Radiance Lighting, lighting is important. To have the most effect, I use a closed room with no windows and doors for outside lighting to come in. I like to use two to three lights, one above, in front, and behind the sim. Play around with them to get a setting you like.
The lighting's strongest when your sim is very close to it (do be careful, as it can get too strong)...
...and weaker as the sim is farther away. It's a little too dim as I took away one of the lamps to prove a point.
Tip #5: The green and blue neon floor lamps are cool-colored, so they add a nice soft lighting. Plus, since they're straight they can be hidden behind the sim :)
Part II: Posing
Tip #6: BoolProp is very helpful (though I don't use it often). You can find the cheats on the Sims 2 LJ community since I can barely remember them myself.
Tip #6a: I use animation hacks. I have Squinge's Hula & More hack (found here: http://www.insimenator.net/showthread.php?t=9544), Shakin's Model Animation Changer (found here: //shaklin.net/) and Aikea Guinea's animation paintings (found here: http://www.sendspace.com/file/msmvzf).
Tip #7: Experiment with every animation. You can get great shots from the most random animations like..
...finding out you're being cheated on...
...falling from the sky..
...meeting with the matchmaker....
....rejecting a vampire bite...
...and some created ones.
Part III: Photoshop
Tip #8: Always sharpen before you size down the picture. You do lose some quality when you don't.
If you can't tell, neither will anyone else. (P.S. You can tell in the difference in the eyelashes)
Some of the pictures I used more photoshop on (the being cheated on, and the falling out of the sky one), but don't edit all of the pic- you want to enhance it, not make it so blurry you can't tell if it's a sim or a doll. I just blurred around the lips, sharpened the whites of the eyes, and colored in the eyelashes. A little work goes a long way.
For more Photoshop tips, check out Sisaly's (found here: http://forums.sims2community.com/showthread.php?t=24516) and applesoveroranges' (found here: http://forums.sims2community.com/showthread.php?t=26632) guides. They're pretty good, and simple enough to follow.
If you want more advice, just comment here and I'll edit this post to add more.