Here is a quick and easy way to make those seamless curves in photoshop.

1. Open up a new photoshop document 400 by 400 pixels with a white background. Now make sure you have rulers and grid turned on to make the curves prefect.

Tip - To turn on grid press (CTRL+') and to turn on rulers press (CRTL+R)

2. Now select the pen tool (P). Click on one area of the page where you would like to start your curve, placing it on the edge of the grid where 2 lines intersect is best, to make a prefect curve.

3. Now click on a second point of the grid (don't let go of the mouse button just yet). Now while holding down the mouse button, drag the cursor just right of the point to achieve the curve you want.

Tip - Make sure Paths are turned on in the pen options up top.

4. Now that you have the curve you desire, it's time to actually make the line. click on the brush tool and make sure the 1 pixel brush is selected. The foreground color should also be the color you would like your line to be. Select the pen tool and right click on your line click stroke path. Select Brush not Pencil and click ok.

5. Now that you have your curve, right click again on your path and simple hit delete path. Your curve should now be drawn out.

Tip - To make the curves less jagged simply add a 0.3 gaussian blur. (Filter -> Blur -> Gaussian Blur)