I flew around with the A300B4 for 2 years. I have a few tips but here is my first. It is more for guy's who will be new to the job as I'm sure the guy's who have done it already know.
Most of the time when your the crew mech you spend the flt in the cockpit for various reasons. Monitoring your aircraft, learning it,there's no space in the back or boredom. On occasion you will have faults in flight and the occasional more serious problems, this is where there is a possiblity of a problem.
In most situations there is a check list the crew performs for any abnormal situation, the checklist is there for safety and is procedure it should not be deviated from. On some occasions with a mech in the seat beside them I have had crews omit this and start asking me what I though and what we might do. The correct reply should be do your checklist and we can talk after. Believe it or not I have been asked these questions while on descent and compressor stalls banging away,which lead to an eventual flameout.
Basically when your an onboard mech you have not done the emergency training,the crm ect... if the crew incorporates you and deviates from procedure they can get distracted and make a mistake or worse.
I have noticed many times that there seems to be a false sense of security with the mech in the cockpit. We may know the systems and be able to help be we won't be doing much at 35,000 and 250 knts.
So I guess everyone has their job at certain stages and is trained for it, so let each person do it to the fullest and with the utmost concentration.