What are some tips for soldering brass fittings to a copper pipe?
Very Important Make sure the joint is really clean when you solder - so use emery cloth or sandpaper to remove any contaminants in your solder joint and flux the joint prior to heating.  I like to wipe the joint with acetone before applying flux but I'm anal retentive like that.

Also you might want to consider using MAPP gas vs propane - those brass fittings take a lot of heat before you will get solder to flow into the joint.