I agree with bjchad that for your early attempts, you may want to choose a thread color that is the same or nearly the same as your background, in a 50wt. I think they look good for a first attempt! Here are a few tips that I've found helpful since I'm also trying to work on my spirals (they're the one shape that's still vexing to me):
1) Definitely practice drawing them on paper over & over again until you understand how to navigate to the center of the spiral and back out to where you want to be
2) Try doing spirals in a line instead of in every which direction. It's easier & will help build consistency as you can compare one to the next.
3) Consider marking your design before you start quilting (or at least mark dots at the top, bottom & center point of the spiral). You don't need to hit the line exactly, but it will give you a good reference point so you don't have to try to imagine the shape & quilt it at the same time. Once you've got the movement down, you can start practicing without any markings
4) If you hesitate for just a second at the inside tip, you will get a point rather than a curve
5) Just keep practicing. They will get better & better the more you practice!
Also, it's entirely a design choice, but many times when I see people use pebbling with this design, they will finish the design motif then echo 1-2x around the entire motif & then begin their pebbling. It helps to define the motif a bit more clearly and gives you a way to get in and out of the spaces without crossing over existing lines. Just a thought. I've done it both ways & there is no "right" way to quilt.