1. Balance all Motors - dynamic - you must do this step. see here
2. Balance all Props - static on a prop balancer and also dynamic by running them to check vibration and eliminate it - if you have not balanced your props you are an ameatuer hack - sorrrrry - but it's true - you will create a raft of problems for your IMU and your video productions - just do it and accept that this is part of multicopter culture and setup - Do not bullshit yourself.  For Folding props make sure they are exactly the same weight and have the same friction setting - pivot with the same forces both sides
3. Check all Prop adapter collets and shafts are true - not bent - check each prop & motor combos are balanced and vibration free when running
4. Mount all sensors on gel or PU gel  - always - we recommend PU Gel or high end Helicopter mounting tape for the IMU - do not use the dji tape supplied - we dont
   Be sure to separate all electronic devices at least 90mm from other devices - Do not place the CPU or the IMU up close to ESC, GoPro or FPV gear!! Bad idea.  For dji pilots - > Monitor the Pilot LED lights at all times especially dji products for "white Sparking" in flight - ignoring or failing to understand that white sparking is serious warning will lead to a pilot failure to respond and centre all controls -> crash - this is a sign you have a bad vibration issue - so abort - land and fix it.. 
5. Mount all GPS/MAG pods well away from all RMI ESC and the main LiPo - NEVER use the DJI lollypop stick - but make sure it not blurred from vibration - ie vibrating so much it looks blurry  - use a small analogue compass to detect EMF fields - do not put your compass in a vib zone or where it gets interference when the copter is running
6. Solder all wires with solder flux paste amtech 223 - don't use cored solder or weak soldering irons - we use a FAST 80w iron with a very large copper tip - the s900/z15 pulls ±37amps in hover at 25v  - we used XT60 (60 amp rating) but we take care to size them to make sure they are tight (not the plastic - the pins) - make sure the soldering is perfect either way by using flux paste and a really big (fast) iron head. XT60 can handle short burst of high temp but not long slow heat from small irons. NEVER USE Y Cables in a Power train - solder dual redundant cables to remove any single point of failure in the supply lines
7. Set all failsafe channels and test them LIVE to check the stability of the RC signal - we set up a mix, to shift the modes channel to failsafe - with a switch
8. Do a proper RC range check of the RC radio with all FPV gear powered on - dont get lazy and skip this check - its very important
9. Test all FPV gear at 200m on the ground - establish which of the 8 freq. is the strongest at long range - Do not mount FPV antenna near RC in or near carbon
10. Check all push connectors are firm - loose or soft connection cause engine/esc failure - for S800 and RC connectors use contact-X on all push on RC connectors - Esp in coastal zones
11. Check all data line are 100% secure - tied but not pulling - For dji PCB pins - check none are loose or wiggling
12. Check all props for fatigue cracks or stress fractures - understand that copper wires - bent back-n-forwards can crack internally inside plastic booms
13. Do a pull test on all Props before every flight to check security - for s800 check the props tip-plane-path from the side
14. Do post flight temp checks on all ESCs and all motors
15. LiPo's - Telemetry Monitor every cell of the LiPo in flight with Quanum or similar - do not rely on total voltage alone.  The LiPo must be a significant proportion of the total aircraft weight (this can only be achieved if the airframe is perfectly matched)  - LiPos which are TOO SMALL will droop on take off, cause rapid voltage loss and overheat the LiPos, reducing their life drastically - LiPos which are too large will overheat the motors in proportion as the motors will strain to carry massive LiPos or payloads - The ideal LiPo size ratio to AUW is 30-33% of total AUW aircraft weight (with camera/gimbal) - the ideal payload (Camera+gimbal) is 15-20% of AUW. So a 1.4kg aircraft will have 420g LiPo ; a 12Kg copter will have 3kg LiPos. Take steps to cool LiPos which are being hammered - this means heatsinks or fans or lots of airflow over them.  (ie hovering over the desert with LiPos getting no airflow will trash your LiPos)
16. Route all data lines well away from ESCs - for ESC this is easy as the ESC are under the motors
17. Operate the CPU/IMU on a dedicated 5v DC bus for large multicopters >8kg - this isolates DC spikes from motors from effecting the CPU
18. Check all wires for airframe shorting - dull down any sharp edges on carbon with a diamond file - pinched wires, cut or chaffed wires will cause major issues - check 
19. Check the airframe is grounded to prevent static entering data lines - props create static - it has go somewhere
20. Separate all RC receiver antenna wires by 90° and keep away from carbon structures - use long antenna -  dont tape antenna near carbon, or carbon landing gear
21. Operations in marine environments - sea salt will oxidise connectors and prevent proper connections - it will also bridge adjacent connectors and create short circuit risk - very easy to guard against by using inox - you must do routine maintenance to prevent damage from salts if you operate in these zones - For all exposed or push connectors - ie all RC plugs - clean them with ACETONE first - dry - then use marine grade corrosion protection such as Inox or Corrosion-X or Deoxit Gold similar sprays to prevent corrosion - less is more - blow away excess with air gun nozzle
22. Ground your aircraft at the slightest hint of abnormal ops of any component, such as motor hesitation or any other sign of human error (a loose wire noticed) or hanger rash
23. Remove all risk - if you fly over a crowd of people the risk is falling - so don't fly over crowds or people or the ocean -   Wear safety glasses - props can cut your hands or EYES !
24. Program and test GPS return to home - even A2 or Wookm has no collision avoidance - be sure to set 50% throttle immediately it start to RTH - be 100% familiar with GPS over ride procedures and fly on MANUAL if req'd
25. Never assume - problems come in groups - its never just one thing, if you find a problem - look again.  Make sure for Hexii that you understand IOC engine failure procedures that it will spin and possibly descend - understand that an OCTA will crash if the IMU is vibrated too much or an autopilot brown out occurs -
26. Check the voltage of your Main CPU rail is exactly 5v - use multiple C1 caps on the 5v DC rail
27. Use loctite - Make sure the entire airframe is 100% rigid and secure - vibration can loosen flight critical parts. Loctite must be heated before your try to remove a screw withit - and can be cleaned and disolved in Acetone
28. Use a checklist and stick to it - all Pro-pilots use checklists - look for problems - Especially vibration - DO an IDLE vibration check before take off !  on every flight !
29. Make 100% sure you ALWAYS see the LED signals - this is vital for "White Sparking" observations - if you ignore White sparking warnings via the LED  - you will suffer consequences
30. Calibrate all sensors - often.  The GPS pod also contains a delicate MAG Compass - calibrate it in away from metal - all metal - including cars and houses.
31. Weigh your Aircraft - use different GAINS for different weights - carry a scale - check the weight.
32. Set the C of G assmetrically - never in the middle - the goal is to force all the engines to run at different RPM in the hover to PREVENT RESONANCE occuring - so set the C of G 10mm forward and to the side 5mm at least
33. Carry spare parts - that way if you have a problem - and are under the hammer to get the flight done - you can safely install the spare part first in minutes - then fly.
34. As the pilot in command EVERYTHING is your fault - if anything goes wrong - YOU as the Captain are responsible afterward.  Posting up your sad story on a forum wont change anything "after" a crash.

Regular safety tips are published here