How can I get a smoother finish when painting trim?
Getting a smooth finish without purchasing an expensive HVLP sprayer is fairly easy.  Start with any good grade paint, and treat it with Flotrol acrylic additive at a rate of about 1/2 pint per gallon and mix well.  If you get a real thick paint, you can increase the amount of Flowtrol up to one pint per gallon max.  I use and recommend Purdy brushes.  For trim, I like a sash brush, (angled style). Be sure to pick out a brush for latex paint, usually a combo of nylon and poly bristles. The ends of the bristles need to be very tapered and soft. Check out the Purdy ExtraGlides for latex. 

Your technique is important also.  Always try to finish with long smooth strokes only using the very tips of the bristles.   Properly treated paint and the right brush used well will give you fantastic results.  Good luck.