I've tried every conceivable position and depending on what you want, different positions give you different results.
Standing - Many people say they can take more in this position. It is probably true but remember that it will not flow as far and more will stay in the descending colon. The only problem with that may be that it may cause more pressure and may stretch the lower colon more. Whether that is a problem or not I don't know but I don't want to stretch out my colon any more than I can avoid it.
Knee/Chest - The best way to:
1) Get the fluid up as high as possible
2) A tie with lying on R side for volume
3) Take it as fast as possible.
Lying on L side (Sims position) - Best way to retain as much as possible
Lying on R side - Best way to get large volume up higher through the transverse colon and into the ascending colon. Slightly harder to retain than Sims position.
On back - Gets lots of volume high up but harder to hold for some reason. Also hardest to get up safely.
All fours - easiest to modify to knee/chest or rolling to one side. Good for volume and height.
Upside down (on an inversion table) - You need to work up to get volume but easy to hold. When you revert to standing you better be close to the toilet and have the floors covered.
Did I miss any? My fav is knee/chest or back. With my wife all fours. Need more of those.