The best practice is a little bit of dawn dish soap, baking soda and water! It seems odd that windex or glass cleaner wouldn't be used, but the vinegar in the windex oxidizes the solder and will weaken the piece over time.

I suggest commercial grade window clings,, or using a small nail and using fishing line to hang where you want your stained glass is also a very safe bet.

I can not take any liability for drops, breakages or likewise after the stained glass leaves my hands.