L.C.
H.,
Have you consulted with your ped.? My girlfriend's daughter had this problem, and she was defecient in ADH- anti-diuretic-hormone. she was Rxed nasal spray and it helped.
I was a heavy sleeper and just didn't wake up. I was 4-5 yrs old. I had on accident at 13 yr, I was dreaming I was using the toilet. being a sleepwalker, and heavy sleeper, my mom understood and kept the embarrassing accident a secret.
All the other suggestions are good (limit H2O, go before bed, plastic etc). If you make these changes and it continues, or the next ped. visit, I would bring the issue up.