I'll pass this on, because it was super advice for me when we used cloth instead of disposables. It's really important to get all the soap out, because it can irritate the baby's skin. Most people will tell you to use multiple rinses. For a much easier time, put distilled white vinegar where you would usually put softener (don't use any softener -- it makes the cloth less absorbent). That puts the vinegar in the rinse, where it will neutralize any leftover soap so it can wash out. Do a second rinse (if your machine has some kind of quick rinse cycle, use it) and voila!

If you have outdoor drying space and the cloth is white, this method has the second advantage of bleaching out any stains without using bleach. (Bleach is another thing you should avoid.) I'm not the type to be picky about having whites that are whiter than white, but we used cloth for about 3 years with Bigglest & the cloth came out the other end still bright white. Very different from when we put them in the dryer.