Laundry Detergent

I brought half a bar of detergent soap like this: http://www.amazon.com/Octagon-Purpose-L ... 754&sr=8-2

When I left it behind at one of the albergues, I easily found a new one in the shops along the way.

We took 2 sets, quick drying. Washed with whatever soap we had (body, hair & clothes).
But drying is your big issue, try get dedicated trekking gear, quick drying.
Probably need extra socks, we took 4 pair, (socks will usually dry on the back of your pack).
Buen Camino
Col

I had two sets of day to day clothes, but three sets of underwear, socks, and sock liners. So at a scratch I could wear the same outer clothing two days in a row, but had clean underwear.

Laundry detergent is something I didn't bother with. I just had a bottle of shampoo which worked well for washing both hair and body (instead of a bar of soap), and washed clothes beautifully. This saved me carrying three separate items for washing.

Some albergues have washing machines and provide small quantities of detergent for pilgrims to buy.

Trudy

You can buy a bar of Fels Naptha in the hardware store in SJPP or in just about any tienda along the way. One bar can be cut into 3 or 4 pieces and shared. It's made for cold water handwashing and does a great job.

I have always used the same soap that I use for hair, body and laundry - a bar of toilet soap, and Dove is the most common in the shops on the Camino. Sure your clothes get to smell of toilet soap after awhile, but that's nothing to stress about. I didn't find anywhere the small packets of soap powder you can but anywhere in Asia. It's a pity the albergues don't make up small quantities to sell sufficient for one or two washes.
Cotton tee shirts are slow to dry, so trekking gear is much the best, and I have found the best to be Iceberg - no odour, quick to dry, warm in the cold and cool in the heat. Iceberg are made from Super Fine Merino wool, and although expensive, they are a good investment. Last year in late October I had snow and the 10 days of incessant rain, so drying was a problem, and the utmost luxury for me in Santiago was to wash and then wear clean and dry clothes. It was bliss. But that was after 5 weeks of no rain, and the year before with a few very short showers
Buen Camino David

In Spain in any supermercado you can buy a bar of jabon de Marsella, aka "Legarto." It washes everything, it rinses well, it´s cheap, and one cake can be shared among several people. Just make sure you get the brown "people" soap rather than the green "floors and walls" detergent -- it will make you cleaner than you want to be!