I doubt that the conversation in the garden included much of what we now call prayer. We know that God is closer to us than a brother. Indeed, through His Holy Spirit He dwells in us. We know from Jesus that God is not impressed with repetition as a method nor is he responsive to Prayers for public consumption or as an expression of arrogance or greed. Surely, he simply wants us to talk to Him.
Fellowship demands that the method of conversation result from the degree of closeness felt.
I expect God wants us to be honest in our prayer and comfortable in our own language. I don't think fellowship with God requires some sort of physical posturing in order to be effective. I'm sure that He appreciates our believing that He is and that He is waiting to reward our belief. Jesus' teaching on prayer is a glorious model of simplicity. What physical position should I be in when I pray' Any position you wish.