1. I really should... 2. I promise that I'll... 3. I swear I'll / I won't... (a very strong promise) 4. No matter what happens, I'm going to... 5. Come hell or high water, I'll... #4 and #5 express your determination to do something, even if obstacles appear.
After all above precautions it is still very important to beware of what you write in the messages and what impact would it have if it fell into the wrong hands. One way of increasing the security of information exchange is to develope a code system for sensitive information exchange, so you would not use real names of the people, real addresses of places, etc.