I always get to the point where you can’t really feel things touching your body, or you can but your senses are dull, yet when I get any sort of dream image, I focus on it, and then it fades, no matter how much I try to stay focused. This happens a few times before I get really uncomfortable, and my muscles get stiff, and I move to get into a different position and lose the numbness in my senses. Anybody know what I’m doing wrong? Also, alot of the time when I realize I’m dreaming during a dream, the image fades away and I try the same way to keep myself focused on it, but then I get to a point of total blackness and my real senses kick in again. I usually stay still for a while hoping I can get back into the dream, but it never works.
One thing that may help you is relaxing your body before you try to rest, this will help prevent stiffness and discomfort.
Try this before you sleep.
Lay down and get as comfortable as you can.
Now tighten the muscles in your feet and calves and hold it for a few seconds then slowly relax them, repeat this with your upper legs, hip and butt, stomach, chest, arms and neck.
If you are more visual in nature try what I do which is imagining a line of relaxing energy slowly moving from your toes all the way up to your head, I usually go slow with this and I can physically feel the parts of my body relaxing as I slowly move the line up.
Don’t go too fast, if you don’t feel yourself relaxing then you are going too fast.
This has always helped me though, I can lay still much longer and my body is so relaxed I don’t really feel the need to move or anything.
Hope this helps.
Taking care of your body ergonomicly and hyghienicly and good movement, nutrition and sleep (amount and normal bedtimes) will also help very much. 
It has happened to me as well. The key is to undirectly focus on the visuals and to allow yourself to go even deeper into trance. By undirectly focusing, I mean not trying to decypher what you’re seeing. Don’t try to grasp them, let them go when you feel they’re about to end. These scenes are very short at the begining anyway.
When you feel like you absolutely need to move, don’t! It’s a trick your body uses to make sure you’re really sleeping before shutting off. A little tricky, I know, but it gets better with practice.
Make sure you don’t get too excited, and stabilize yourself and the environment with affirmations such as ‘Calm down, NOW!’, ‘Stabilize dreamscape NOW!’ and so on.
I hope this helped a little. Good luck!