Here are two great stretches to open up your upper back toprepare to do Wheel:

For this first one you can start in Hero, Crossed Leg Pose or simply stretch your legs straight.  Using 2 blocks, make sure one block is under your upper back, between your shoulder blades.  The other block is under your head. You can start with both blocks on the high side, if possible.  Hold for 1 minute with the arms overhead holding opposite elbows. If you place the block under your head strategically, you will be able to rest your forearms on the block.

Then lower the head block to the medium height.  Switch the way you are holding your arms and, if your legs are crossed, change the cross of your legs.  You only need a small portion of the block for your head to rest on. Let your forearms rest on the block, too. Hold for 1 minute.

Then change the block to its lowest side, if you can.  Again change the arm cross and the leg cross.  If the head does not touch the block, then either put something on the block to make up the difference or stay at the height of the block where the head is still supported.  Hold for 1 minute.

Finally, I went to no block and head on the floor.  Go only as low as you can keep the headsupported on something.  Hold for 1minute.  When finished, come out slowlyand come onto all fours and do a couple of cat and cow stretches. 

For the second one, place two blocks in front of a chair so that you can sit on the blocks and lay your upper back over the chair. Make sure that your head and shoulders are resting on the chair.  Lift your buttocks up so that you are in a straight line from your knees to your head.  Thread your hands through the back of the chair and either hold your arms straight and squeeze a block between your hands or bend your elbows and grab the back rung of the chair.  Keeping your shoulders and head on the chair, begin to lower your buttocks toward the blocks without tilting your chin up. You end up using the front edge of the chair seat to leverage your upper back open.   Hold for about 10 seconds and try to find 2 other spots on your back to leverage over the front edge of the chair.  When finished, release your hands, slide your buttocks carefully down to the blocks and sit up.