Here you will learn how to smooth a shape. Remember that this technic also round the shape as well as it smooth it.
1. First of all, select with Ctrl + Click the layer that contains the shape so that only the shape you want to smooth is selected. The mouse should look something like this when you Ctrl + Click on a layer:
2. Add a layer mask by clicking on this icon, which is just below the layer frame:
The layer should look something similar to this:
3. Now got to Filter -> Blur -> Gaussian Blur and set the Radius to 1~3 pixels or even more depending on the size of the shape you want to smooth. Now, Click OK
4. Now RIGHT CLICK on the Layer Mask and choose "Apply Layer Mask".
And you are done.
There is one other technic which smooth you shapes nicely, but this technic doesn't only smooth them, but it round them aswell.
1. First of all, select with Ctrl + Click the layer that contains the shape so that only the shape you want to smooth is selected. The mouse should look something like this when you Ctrl + Click on a layer:
2. Press Q to enter the quick mask mode.
3. Now got to Filter -> Blur -> Gaussian Blur and set the Radius to 3~6 pixels or even more depending on the size of the shape you want to smooth. Now, Click OK /!! Note that the bigger the radius pixel number is, the rounder you shape is going to be. /
4. Image -> Adjustments -> Brightness/Contrast and set the Contrast to something between 70% to 95%. /!! Note that the bigger it is, the bigger, and rounder your shape will be
Now, you have to set the Brightness. The lower the Brightness is set to, the thicker your shape will get, and the bigger the Brightness is, the thinner your shape will be. This is the settings I used:
5. Now press Q to quit the quick mask mode, and then fill the selection.
More Tutorials: 
