With the furnace running constantly in the winter, my plants get pretty dirty. Here's a "before" shot.
I could spend hours washing each leaf, but I don't have time for that. Instead, I occasionally (about once every 4 months) place my plants in the shower and turn on the water. A minute in a lukewarm shower cleans them well. I let them air dry before placing back in my living room.
It's really important to get dust and dirt off of your plants. If you don't, the plant can't breathe and the leaves will dry up and fall off. Then the plant can't make food and it dies.