After being active in the online lucid dreaming community for almost half a year now, I see that both lucid dreaming newbies and experienced oneironauts have at least from time to time problems controlling and manipulating their lucid dreams. I, too, have problems now and then. Lately, I've been paying attention and working on overcoming these problems when they occur. I've found some interesting results, so I want to share them.
Please keep in mind that these techniques might not work for you even though they work for me. However, they are sure worth a try!
First off: there are lucid dreamers that will tell you that some things are impossible to do in lucid dreams. The most famous myths when it comes to this is the idea that you can't manipulate light, you can't get digital clocks to work, and you can't get text to appear stable. Don't listen to them. Your only limit is your imagination, and also what you
think you can do.
Things that hinders you from gaining controlYou've all watched the Matrix. You have seen how Morpheus teaches Neo to free his mind, to know that everything is possible, when you free yourself from doubt, disbelief and fear. This applies to lucid dreaming. As an example, newbies often post that they are unable to fly. After talking to them, it appears that the most common reason for this problem is that they have some level of doubt that it will work.
Trying too hard is also a hindering. This is my most common cause of control problems. Frustration often follow this, making it even harder, and is also a control stopper.
So, what do you do when you find yourself unable to gain control in your dream?
Free your mindIt's all about believing that it's possible. Don't think you can't do it, because if you think that, you can't. Lots of other lucid dreamers can do the same things that you want to do, so why shouldn't you? Even when awake, tell yourself that you can do it.
Don't try too hard, don't get frustratedFlying or levitating something is a great example here. No matter what you do, it doesn't seem to work. So you try new methods. You try harder. And harder. Not only does too much effort make you risk waking up, it will make it even harder to do. I'm also talking from experience here. I've woken myself up plenty of times while trying to teleport because I put too much mental effort to it.
Try a more relaxed approach. Say you tried to levitate a box. You used the approach as mentioned above. If you haven't woken yourself up on accident by now, you are getting frustrated. And I tell you, the more frustrated you get, the more impossible will it be to control your dream. Relax both your mind and dream body, then try again, gently. Give yourself some time. You are now more likely to have success.
Practice makes perfectThis especially accounts to newbies. Even if you still fail your control attempts, know that there usually is a learning curve, and if you find out that you have more of a learning curve than others, don't let that put you down. When you then master the skill, you have come a long way, and you should be very proud of yourself! Doesn't accomplishment feel even better when you have worked even more for it?

Whether you're a newbie or expert, I hope you get something out of this thread. Feel free to post thoughts, comments and personal experiences.