Are you just doing one floor, in between joists?
The product I have used a few times is the Rehau heat transfer plates. They are made of thin gauge aluminum and you just staple them up with a staple gun and leave about a 4-6" seperation between plates throughout the length of the pipe. They work very well.
The company I work for normally uses Rehau O2 barrier pipe for our radiant, and it works great for in floor radiant that is encased in concrete or a concrete subfloor, but it's not so good for staple up. Reason is we had one job where we used it for a staple up retrofit and the O2 barrier for this pipe is a bit squeay due to expansion/contraction. I would suggest going to Wirsbo HePex or another type of Oxygen barrier pex for this application.
The heat transfer plates will help the pipes conduct heat better and spread the heat out better as well. As far as what to put underneath its up to you but Reflectix or Roxul will be fine. IF it's over an unheated garage, crawl space, or exposed area I strongly suggest you beef up the R value of the insulation below your radiant. I'd say R-12 if there is a heated space below and minimum R-20 if it's exposed below.