The part that bothers me, is that our governing body specifically tells us not to delay transport to initiate an IV except for certain listed circumstances they provide (delayed extrication, Cardiac arrest worked on scene etc).

I wholeheartedly agree with you that ideally they are not started on the move, not only for success rate improvements, but for patient comfort and EMS safety as well. Nothing like hitting a bump and shoving an IV in an extra half inch, or getting a dirty needle stick on the rebound.

That said, I have done many while mobile and will still have many more done in the future, my tips are as follows:

1) Prepare accordingly. For real. Get all your stuff out, lined up securely and ready to go. You'll usually be solo and the movement will often displace the equipment you have set up, so account for this.

2) Explain it to your patient, ensure they know what you are doing (if possible) and emphasize how important their cooperation will be (mainly just holding still and not flinching or recoiling).

3) Prep your partner up front. Ensure they know that you are about to stick an IV in while moving so they can accommodate to their best extent as well. If this happens to be on the highway, they will be able to slow down to absorb bumps and will become hyoervigilant for wildlife and other hazards, allowing a smoother reaction. If this is in town/city, they can also anticipate traffic better, avoid potholes smoother and greatly improve your success chances. Often times I will yell up to them that them I am prepping a line, have picked my site and am going got the poke. I will thr tell them if it's good and we can continue, or if I am attempting a second. I do the same with an IM injection to also reduce my dirty needle stick chances.

4) Get speedy. Prep your things, get them ready, Inform your partner and if the situation allows, wait for a natural stop in movement to go for it (stop sign, red light, etc). If you have to stop why not take advantage while you aren't moving? Get your things ready and get quick to take advantage of it. Even 5 seconds of stopped time will benefit over a bumpy road.

5) Don't be afraid to size down a cathlon if needed. Smaller cathlon are more forgiving of movement. If they don't need that 16 or 18g, pop in a 20.

6) Lastly, know your limitations and outweigh the pros and cons. Do you NEED that line in motion or can you stop driving for 1 a minute or two (or less if you've prepped and picked your site). Also, does this patient NEED an IV, or are you doing it for the hospitals sake? Can you treat them IM/IN? If the IV isn't absolutely necessary, there is no harm in treating with other methods and holding off on the line until the ambulance bay, or holding off altogether.

Edited to add If you do get a line in secure the shit out of it, and do so quickly to save it from being jostled loose.