1st real dry suit experience

[scubagrrl]

So...we did our pool dive last Thurs night.  I had problems apparently because my rental suit is too big.  The exhaust valve kept turning to the back of my suit where I couldn't reach it.  Made for fun trying to scramble for that while trying to somersault around to get air out of my legs.    Anyone got any suggestions on something I can do to keep that thing in place for our lake dive on July 14th?  I think our shop only has something like 5 suits and my son's using the medium, so I'm stuck with the med/large.