I ran across some foundation tips for you do-it-yourself kind of makeup wearers out there.
Wear a foundation that's a shade deeper than your actual skin tone.
I know what you're thinking, and I thought it too (at first). But think about it--have you ever noticed that in photos your face sometimes appears lighter than the rest of you? It has to do with reflection of light off of makeup (that's how it hides blemishes). So when you're having your portrait done, even though it might make you cringe in front of the mirror, go a bit darker. You'll be happy you did.
Keep powder off of pimples.
The problem with hiding blemishes is that you usually end up with a tiny cake of makeup that sticks out more than the pimple originally did. So keep your coverage simple. Apply a tinted moisturizer. Then--on the blemish--lightly blend concealer on top of it and out. That's it. And remember--whatever you do--resist the urge to powder.
If you have a blemish that's just too heinous, I can fix it in PhotoShop and no one will ever know.