My luck has been this: The compass in the actual app works well, but if you're not out in the wide open, it will take some time to figure out where to point you and even then it might be off. What I do as far as that is concerned, get to the general area, set my phone down and start searching, come back a minute or two later and see what it says. At that point it should have calmed down enough to help you out.
A GPS device is going to work much better than the iPhone for getting you close. Usually my iPhone does the trick, but there are areas out in the middle of nowhere where I wish I had a legit GPS device and not another DNF on my list.
Like CanDMan47 suggested, start small and go bigger as you learn the tricks. If you're really stuck on something, 75% of the time you wouldn't even know you were walking on top of it, you just dont have the experience to find them, it's not the GPS device. I still have that issue. I'm under 100 finds and I keep finding new things that I'm surprised I even find.
good luck!