Perfect Harp Playing?

First, make sure you're making large movements without getting overexaggerated. Second, I'm not sure if this is how you're supposed to do it, but I've found it easier to be precise if you hold the Wiimote rotated 90 degrees (so the buttons face to your left). You can also try recalibrating and recentering the Motion Plus just before starting the minigame, and see if that helps.

I found this minigame to be very glitchy. Basically, do your best to follow the hands of the audience for most of the song. You do not have to go the same direction as them, you only need to match the rhythm. I also found that I had to make less dramatic movements. If you go too far, Link starts playing back down the harp for some reason (hence the glitchy comment earlier). Instead, fail to the point of not making it to the last strings on each side to keep the rhythm going. After a few tries, you should get the heart piece.

Just be the arms. Don't move to fast as it sometimes slows the tempo. After a few tries yiu get used to the song and do better. How the wiimote as shadow said. Good luck!

Unfortunately, the harp is very hit or miss and seems to work better for some people than it does for others. There is no guaranteed way around this, although if I am wrong I would love for someone to tell me.

There are, however, a few things you can do that might (emphasis on MIGHT) make things go smoother. For one, concentrate on the ring that's expanding and contracting, not on Link, the harp, or your Wii Remote. This helps immerse you more and makes your movements more precise. Secondly, as Shadowlynk has pointed out, tilting the Wii Remote so its buttons face to your right or left may alter the way Link plays, though for better or worse is hard to predict. Thirdly, any time you DO get Fi to start singing, try and do absolutely nothing to change your speed or movement style. That's easier said than done and should probably be pretty obvious, but hey.

For a more in-depth analysis, you can check out my Motion Controls Guide in the FAQ section. It should be more informative than this post here.

Oh, I just realized I provided advice for the wrong situation. Sorry.

All I can really do is emphasize what you've already been told. I'm 99.81% sure you're referring to the harp mini-game in Skyloft's Lumpy Pumpkin, which involves an "audience" as your reference point rather than a growing and shrinking ring. The same principles apply, though: pay attention to the audience as opposed to anything else, and try and get used to the rhythm. It's hard to predict their movements every time, but you should be able to react pretty quickly to any changes.

Despite how quickly the audience shifts their arms from left to right, you still want to play somewhat slowly. Ideally, as soon as you reach one end of the harp, the audience will shift again. Make your sweeps with the Wii Remote wide and flowing so mistakes blend in more.

If worse comes to worst, it may be time to either recalibrate your Wii MotionPlus and see if it solves the inconsistency issue, or just try, try again until you get the desired reaction. Again, the harp is very hit or miss and doesn't work well for me, either. It could be we simply have to live with "good enough."