A few things about felting that I've learned along the way:

1. You don't need hot water, warm water will do.

2. Just a wee bit of shampoo or mild soap will do -- like maybe a tablespoon or so.

3. A top loading washing machine is one of the fastest felting tools on the planet -- check your bag every minute (I'm not kidding) until it is the size and shape you want it to be.

4. A front loading washing machine is not one of the world's fasted felting tools -- but you can get great results if you follow a few simple principles. Here's what I do on my Maytag Nepture (you may need to adjust this for other brands):

• set the agitation level to cotton/sturdy
• set the temperature to hot/cold.
• 15 minutes in a Neptune equals 1 minute in a top loader. Check after 15 minutes, then check every 10 minutes thereafter until your bag is the way you want it to be. I wear a sports timer to tell me when to peek again.
• most front loaders lock the door - when it's time to check, just hit stop and walk away for 3 minutes, then open, look, and start again if needed.
• never, ever allow the machine to go into the rinse/spin cycles - this will crease your felt and you'll have to start all over again. Reset the wash cycle if you see you're nearing the rinse cycle.