How does power dissipation for surface mount components work?
SMT components are usually heat-sunk to a copper plane or a copper pour on the PCB.  Ground and power planes are usually the largest copper pours, so it’s nice to be able to heat sink to them.  But that’s not always possible, and additional copper pours need to be created.
In some case certain pins are better for heat sinking than others.   For example, in this power MOSFET, drain pins provide heat sinking.  As far as I know, all MOSFETs in SOIC are arranged like that.
 (source Fig.5 in the application note)
Electrically, the drain is usually not connected to power or ground, and a separate copper pour needs to be created for heat sinking.
The area of the copper pour sets the thermal resistance between the outside of component and ambient air.  Some data on PCB heat sinking can be found in the LM317 datasheet pp. 15-17
Here's my own example of PCB heat sink.  A 7805 voltage regulator in TO-252 (U4) is heat sunk to a 2-layer PCB. Fortunately, the thermal pad on 7805 is connected to ground.  So, I can use the ground copper pours for heat sinking.
Top layer 
Bottom layer 
Manual for the SMPS controller's evaluation board is another source of layout guidelines.  (Not just thermal management.  SMPSes can be layout sensitive.)
Further reading
Fairchild (now part of On Semiconductor) application note Maximum Power Enhancement Techniques for SO-8 Power MOSFETs
TI application note Semiconductor and IC package thermal metrics