Here are some ideas that might help you use a nasal interface successfully:
1. Tongue position. Try putting the tip of your tongue behind your upper front teeth. Then position the main part of your tongue up against your upper palate. Finally, give a little suck or swallow to create a bit of suction. You should now be able to open your mouth while breathing entirely through your nose. Practice this during the day, and see if you can get it grooved in deeply enough to help while you are asleep at night. For some people, this is really all it takes to avoid mouth leaks.
2. Collar. If your jaw tends to drop down during the night, pulling your mouth open, then a soft cervical collar, or a firmer snore collar, can be a big boon. More here:
http://www.apneaboard.com/wiki/index.php...cal_Collar
3. Although chin straps tend to pull the jaw back, which is not helpful, the Knightsbridge Dual Band strap has a design that pulls the jaw straight up, which makes it a possible alternative to a cervical collar.
4. Tape. Some people (including me) rely on tape to keep the lips from opening. (I can't rely entirely on the tongue thing.) When the lips open, all too often that eventually leads to a noisy and disruptive mouth leak. To see whether tape would be feasible for you, I recommend that you invest in a box of Somnifix strips. They are very gentle on the skin but hold very well. Be sure to curl your lips inward per instructions before placing the strip. If these work for you, then you can experiment with lower-cost options. (Note: Somnifix must be having supply-chain problems these days. If you have trouble finding them, you could use any gentle tape. I sometimes use BSN Medical Cover Roll Stretch, 4" x 10 yds, Single Roll, White - 50423. Be sure to fold over one end so you'll have a little tab you can grab to remove the tape.)