I'm working on an image of the Warsaw Ghetto during World War II, inspired by the movie, "The Pianist." When using seamless photo textures, what are some techniques to make the repeating pattern less obvious?

Well.. increasing the resolution is not an option in this case, but that is one way. the less times it is tiled the less obvious it is. Try to use it in small areas, put other objects from the scene infront of it etc. Things like that. you can also manually mix up the texture in photoshop IE tile it manually 6 times or 8 then scribble something on part etc.

Just a twist on Ornlu's thoughts.... Figure out how many times the texture repeats in your image, then go manually tile the image in something like photohop. That way you can add little tweaks here and there.

Yes, both suggestions are very good. When I encountered a similar problem, I did partly what Claymor said and partly Ornlu's. I used some signs and posters to hide the most obvious repeating patterns and tweaked a bit the tex in Photoshop. In the case of your picture above, I think you're already in a good situation: the only repeating I see clearly is the darker row at about 2/3 of the wall. In this case, you can render the wall as separate cubes and apply the same texture by changing the vertical rotation by 180 degrees. And, btw, I think you chose a very good subject. My compliments for that. S.

I'd make an ABC texture out of it -- two slightly different repeating photo textures with a subtle cloudy procedural as the mask. Don't know if that makes any sense, but that's what I would do.

Yeah, make a grunge map of a different size, then they will always overlap at different areas, making it appear less tiled. I can't see the pattern here, so I think you're fine as it is, IMHO.