Until recently I have always mounted my acrylics painted on 240g paper by applying double sided tape round the edges and glueing a cardboard back to the mount.
Now I have read somewhere that the double sided tape should only be applied to the top of the painting to allow it to move. Doing so I have not glued the cardboard or mount card to the back of the mount the way I used to.
This resulted in the painting not being flat anymore and gaps appeared between the painting and the mount.
Having discussed the matter with experienced artists and framers I have gone back to using double sided tape to attach the front and back of the mount firmly to the painting. However, I now fix the top of the painting only to the mount with a strip of double sided tape. The tape overlaps the painting by just 5 mm. This gives the paper room to move sideways and down depending on environmental conditions.
The method should be equally applicable to watercolours. There is more on this subject on Painter-Online.