With a small amount of setup you can create a canvas that will allow you to draw symmetrically. This means that you only have to draw half of a symmetrical image to create a full image.
Open a new document 8.5 x 8.5 inches.
Choose the rectangle tool and set the stroke and fill to blank.
Draw a 4.25 x 8.5 rectangle on the right side of the canvas.
Select the layer you created the rectangle on and go to Effect > Distort and Transform > Transform.
Use the settings shown below and click Ok:
Choose a brush and draw on the right side of the canvas. You will see the stroke you drew reflected on the left side of the canvas.
Keep drawing your design on the right side of the canvas.
Refer to Simple Seamless Patterns for how to turn your symmetry document into a seamless tile.