Are you referring to melt and pour soap projects like Amareluna's chocolate cake* (http://www.craftster.org/forum/index.php?topic=137880.0
)? I'm not sure how Amareluna does it, but this is how I do it:
Basically you're going to pretend that your soap is just like a baked good; if you've ever drizzled icing on a cake or cookies, it's the same concept.
First, make your M&P bar onto which you want to drizzle. Let it set up and be completely finished and/or hard. Then, melt the soap you want to drizzle. Add color and fragrance and stir until soap gets to a fairly thick consistency. You don't want it to be too thin because, much like icing, it won't make a pretty drizzle effect, but you don't want it to get too thick or cool because it won't make a nice drizzle. Once the soap is the desired consistency use a measuring cup that has a well-defined pour spout (I find plastic ones work best, but whatever works for you!) and slowly drizzle the soap over the bar, much like you would with icing. You can do a nice zigzag pattern (simplyorganic's purple bar** at the end of this post is lovely! http://www.craftster.org/forum/index.php?topic=267203.0
), or you can make it look more like frosting like in Amareluna's. Then you just let it setup like normal soap and voila! Because you are pouring so slowly, you shouldn't have a problem with air bubbles, but if you do, keep a spritz bottle of rubbing alcohol handy so you can spray it and burst any bubbles!
*I hope Amareluna doesn't mind me linking her cake, but it always brings a smile to my face!
** Again, I hope Simplyorganic doesn't mind me linking. Her projects also make me smile... and feel very hungry!!