How to glue a LEGO tree decoration in a way that's not permanent, to prevent accidental disassembling?
You can increase the "clutch" power of bricks by sandwiching a piece of paper towel between each connection. Once the bricks are together, cut away the excess paper using a knife.

The effect of this is roughly the same as applying a weak glue. Your builds will not become impervious, but will hold up better to handling and hanging.