It's common to hear about layering, especially with regard to texture, weight or color/metal. All great ideas, but I happen to think this idea was pretty brilliant: the combo necklace, where you take two necklaces and hook their clasps together to make one longer necklace. The idea here is to create a new look with two pieces, and it's almost better if they don't look too "matchy matchy" so there's some visual interest to the piece.
Here's what Redbook (June issue, p.22) showed for an idea:
And here's what I put together from my necklace collection:
I like the chunky-ness of Redbook's suggestion- they key is both necklaces have to be short enough individually so the connected piece is just the right length. I don't think I have a short enough colored necklace to pair with my gold circle chain to make it look just right. But a creative idea, no? I'll keep working on mine, but you should check out your collection to make a new look!