I Increased My Muscle Strength Using Visualization
Mental Rehearsal
Mental rehearsal is a form of visualization that helps you prepare for an event. You visualize yourself going through the motions and getting the outcome you want. The main advantage over physical practice is that you can always experience the outcome you want. You can also use it when you can not do it physically. Athletes often use mental rehearsal to improve their performance.
The visualization should be similar to what you can actually do but a bit better. Day dreaming about having super strength will not help you. Physically practicing will keep it fresh in your mind. When I am visualizing an action like exercise I find it easier to do if I breath the way I would if I was physically exercising. You can improve your ability to visualize with practice. Watching videos of you or someone else can help.

Visualization Experiment
I saw some experiments that showed it was possible to increase your strength by using your mind. In the experiments people visualized themselves doing an exercise instead of physically doing it. As a result of the visualization they significantly increased their strength.
It seemed like a good experiment to try because it would be easy to measure the results and it would not take too long. With actual physical exercise I can notice a difference in performance in a week or less. So I designed a simple experiment I could do for a week. For my visualization experiment I used push-ups for my exercise because they are easy to visualize.
To start I did as may push-ups as I could do while keeping proper form and recorded the results. I do slow and steady push-ups focusing on quality not quantity. For the next 7 days I visualized doing the push-ups. Since I was just picturing myself doing the push-ups I did over twice as many sets as I would have otherwise. To keep it realistic I visualized myself doing 5 more than I could when I started the experiment and gradually increased it to 10 more.
At the end of my experiment I could do 6 more push-ups. I became noticeably stronger within a week. This does not mean that my muscle became bigger or harder. My brain and body could just have gotten better at using what I already had. To see if I could increase my muscle mass with visualization I would need to give up physical exercise for a long time. However it proves without a doubt that I can improve my strength.
Uses
I would not recommend just using visualization. In most cases it is better to do the actual physical exercise. However it is a good tool. If you can not exercise physically you can use visualization. You could also include it as part of your exercise routine. It can help you exceed your current limits by preparing your mind and body.
To build muscle you could lift weights and visualize lifting weights when you wake up, as you are going to bed or on your rest days. I like visualizing in bed just before I go to sleep. For me it works best for short mental workouts. If you want to improve your ability to run long distances you could skip over some of the running and start near your current physical limit.
Visualization is a powerful tool that can be used to improve your strength. Try it out for yourself. You might be inspired to use visualization to improve other things.