First of all, I would highly recommend practicing on a muslin block with scrap batting when you're first starting out. Also, it helps to simply take a pencil and draw different designs freely on a large piece of paper to get your mind and hand used to doing free motion designs without lifting the pencil off the paper (you're trying to mimick the machine here, and since you can't lift the needle and machine off your fabric, try to keep moving without lifting the pencil.)
After you've practiced for awhile, your stitches will get more uniform. You'll get a good idea about how fast you should be moving the fabric under the needle in relationship to how fast your machine is stitching. PRACTICE, PRACTICE, PRACTICE!! Don't give up. Keep trying.
I have a Juki domestic long arm and I love it to death!! The presser foot barely moves off the fabric which helps keep it more uniform too. Hope this helps!