Reverse prayer can be a good way to prepare for wheel pose.
You can practice reverse prayer position (hands behind the back with palms together) while sitting or standing, or while bending forwards and stretching the hamstrings in parsvottanasana.
This arm position can be used to help stretch your chest and the fronts of the shoulders. It's also stretches the fronts (palm side) of the wrists.
Preparing Your Shoulders
The first part of getting your arms into position behind your back is rolling the arms inwards while reaching them back (and out) at the same time.
You could think of this as "internally rotating" the arms.
If you have trouble doing this with both arms at the same time, then practice first with one arm at a time as shown below.
When you internally rotate the arm at the shoulder, the front of the arms roll inwards. This means that the back of the arms roll outwards, away from the spine.
This helps to give you room so that you can bring your hands together behind your back.
Practicing Internal Rotation for the Shoulders
You may find that if you rotate your arms inwards enough, then when you bend your elbows your forearms move behind your back. In this case, with arms rolled inwards, your forearms will be pointing slightly backwards.
I'd suggest that if you can't get your hands behind your back, then practice internally rotating your arms. Rather than holding the internal rotation you practice rotating and then releasing in time with your breath. Focus on feeling your arms and shoulders as you practice and see if you can notice where the tight spots are. Again you may also benefit by working with one arm at a time. Do not be afraid to bend your ribcage forwards as you rotate your arms inwards.
Even if you can internally rotate your arms you may still have difficulty getting your palms together in reverse prayer.
An option, shown below is to make both hands into fists.
When doing the double fist option, you can first have the back of the hands against your back.
Then flip your fists so that the bottom of each fist faces downwards (bottom left two pictures).
Then with both hands in fists progress to one hand open (bottom right). Note that with this option, with just the right hand fisted you could try starting with both hands to the right. Then gradually move your hands to the center of your back.
One you are able to do reverse prayer comfortably, another option for increasing shoulder and wrist flexibility is to move both hands to the side with palms together.
Move both hands to the right so that your hands are to the right side of your ribcage, but behind your back (shown below). Then try the same thing to the left.
Opening Your Chest and Shoulders
Once you can get your hands together behind your back in reverse prayer you can practice moving your shoulders and elbows back.
You can do this using any of the above hand positions. I'd suggest that again you practice doing this in sync with your breath.
As you inhale open your chest (bend you thoracic spine backwards) and at the same time move your elbows back to open the front of your shoulders. Then as you exhale relax your chest and let your elbows move forwards (first two pictures below).
As you get used to the position you can keep your chest open and your elbows back. You can also work at sliding your hands higher up your back (last picture below.)
Reverse prayer can be a good way to prepare and open the chest for wheel pose.