Sometimes I forget to water and the top layer dries up. Since I use my garden hose, I set the sprayer to the shower setting, water for 30-sec, wait about 5-min, then water again. I'll do this a few times until I'm satisfied the area is saturated.
Soap does help. I use Dawn but any brand that isn't to harsh, should be fine. If I'm going 100% organic, then I'll use Yucca extract, which isn't to expensive. An 8oz bottle usually runs about $7-$10 US.
If I'm going away and I'm worried about the soil drying out, I'll lay an extra thick mulch or I don't have that handy, a few layers of wet newspaper works pretty good in a pinch. It takes the beating of the sun, so the soil underneath remains cool & moist. If this happens a lot, I make sure the next year I amend the soil with more coir, shredded newspaper, and water storing crystals.
Another thing to look into is soaker hoses and drip irrigation set to a timer. The slower and longer flow of water prevents evaporation and water loss, while keeping the soil moist more consistently.