First, acquaint yourself with your setup, and make sure you are aware of where the HHO is at all times. Be aware of every hose, barb fitting, elbow, gas pocket and device. Anywhere HHO resides, you have the potential for a flashback. Pay special attention to large pockets of gas in areas like Wet Cell containers and Dry Cell reservoirs. If large enough, these can pack a lethal punch in the event of a flashback. The most vulnerable place in any HHO setup is the point of exit. Avoid flames or sparks directly in front of your exit hose, unless you are using a torch designed specifically for sustaining a flame.
The point of exit isn't the only vulnerable part, though. Sometimes a flashback can occur from within.
In the early days of my research, I once had an electrical short in the bottom of a glass jar cell (cookie jar size). Of course, at the time, I didn't realize how dangerous it was to use a glass jar. I didn't know I had a short until it ignited the gas all the way up through the electrolytic bath and into the large pocket of HHO at the top. The blast blew off the metal lid which was securely screwed onto the jar. The noise left my ears ringing for hours. Fortunately, the glass didn't shatter. I should have taken proper steps to avoid the short, yet had blast relief in place to control the outcome of the blast.