If I'm not mistaken it was Marpa who once famously said something to the effect of: "Just my meditation posture is more effective than all the meditation of all the meditators in Tibet."
Posture (the 7 point Vairocana posture mentioned earlier) is very important to establish before starting. Once you have the correct posture it should take care of itself. If you are fussing in a muscular way for example you probably haven't gotten it down yet. Most important is the spine. Sitting with your butt a little bit higher than the level of the floor or wahtever you are sitting onis highly recommended, that way you can keep the back straight like a stack of coins without muscular issues in the lower back etc. The straight spine allows the energy to flow freely.
As to having trouble with Samatha, well I hear that if you have gone through all nine levels of concentration and acquired flexibility of mind and body, bliss of body and bliss of mind, (the 'fancy things' 5heaps was referring to) and then Samatha or meditative serenity comes naturally.
Meanwhile better do shorter sessions of higher quality of sharp, instensely focused meditation than hours of fidgeting and wandering and wondering whether to adjust your posture.