Sitting between the Two Sajdahs:

"He would lay his left foot along the ground and sit on it [relaxed]"147, and he ordered "the one who prayed badly" thus, saying to him, When you prostrate, prostrate firmly, then when you rise, sit on your left thigh.148 
"He would have his right foot upright"149, and "point its toes towards the qiblah."150

Iq'aa'Between the Two Sajdahs:

"He would sometimes practise iq'aa' [resting on both his heels and (all) his toes]."151

Lengthening the Sitting between the Two Sajdahs:

Also, "he would lengthen it until it was about almost as long as his sajdah"154, and sometimes, "he would remain (in this position) until one would say: He has forgotten."155    

The Adhkaar between the Two Sajdahs:

In this sitting, he (sallallaahu 'alaihi wa sallam) would say: 

1. O Allaah! (in one version: O my Lord!) Forgive me; have mercy on me; [strengthen me;] [raise my rank;] guide me; [pardon me;] sustain me.156 Or sometimes, he would say: 
2. O my Lord! Forgive me, forgive me.157 

He would say the above two in night prayer also.158

The Obligation of Being at Ease between the Two Sajdahs

"He (sallallaahu 'alaihi wa sallam) would be relaxed until every bone returned to its (proper) position" (152), and he ordered "the one who prayed badly" likewise, and said to him, The prayer of any of you is not complete until he does this. (153)