Did you notice when you've drawn a circle or a box and SOMETIMES
you saw an 'un-smooth edge' around it ? This tutorial will show you how
to solve that 'un-smooth edge' become a 'smooth' one
1) Create an image with marquee circle selection tool (fig1)and in the options window de-select anti-aliasing.
(fig2)
Fig 1 Fig
2
2) Ctrl+click that layer, then in channel window.....create a new
channel 
(fig 3). Fill the selection in that
new channel with white, then ctrl+d to deselect it (fig4). 
Fig3
Fig
4
3) Now go to Filter > Blur > Gaussian blur, I used 3
pixels here. (fig 5)
Fig 5
4) Go to Image > Adjust > Levels, grab those sliders at
the far ends and bring them close together, until you have a nice smooth
edge. (fig 6)
Fig 6
5) Still on the channels window, click the "Load Channel as Selection"
button 
(fig7)
Fig 7
6) Go back to Layer window and create a new layer () then
fill it with color (fig8)
Fig 8
Hope it helps :)