Procedure before starting Gayatri Japam

Sage Saunaka has prescribed the following procedure before one sits for chanting gayatri after the water oblations during sandhyavandanam.

आ ब्रह्मलोकादाशेषादालोकालोक पर्वतात्
ये वसन्ति द्विजा देवाः तेभ्यो नित्यं नमो नमः
इति मन्त्रं  ससुच्चार्य नम्स्कुर्यात्सुरद्विजान्॥
अपसर्पन्तु ये भूताः ये भूतः भुवि संस्थिताः
ये भूतः विघ्नकर्तारः ते गच्छन्तु शिवाज्ञया॥
तलेन वामपादस्य भूमौ खातत्रयं चरेत्।
उग्रभूतपिशाचाद्याः ये च वै भूमिधारकाः
एतेषामविरोधेन ब्रह्मकर्मसमारभे
सङ्कल्प्येवं जपेत्सम्यक् मन्त्रार्थगतमानसः
कुशाजिनाम्बरेणाढ्यं कल्पयेदासनं बुधः॥
ābrhmalokādāśeṣādālokāloka parvatāt
ye vasanti dvijā devāḥ tebhyo nityaṁ namo namaḥ
iti mantraṁ  sasuccārya namskuryātsuradvijān||
apasarpantu ye bhūtāḥ ye bhūtaḥ bhuvi saṁsthitāḥ
ye bhūtaḥ vighnakartāraḥ te gacchantu śivājñayā||
talena vāmapādasya bhūmau khātatrayaṁ caret|
ugrabhūtapiśācādyāḥ ye ca vai bhūmidhārakāḥ
eteṣāmavirodhena brahmakarmasamārabhe
saṅkalpyevaṁ japetsamyak mantrārthagatamānasaḥ
kuśājināmbareṇāḍhyaṁ kalpayedāsanaṁ budhaḥ||

First the Brahmana should offer multiple.pranaams to all the gods and twiceborns who are living in the terrain between Brahmaloka to Patala (the fourteen worlds) and between  Loka mountain and Aaloka mountain.  After chanting the manthra  Aabrahmalokaad... (as above). 

Then he should chant the mantra "the devils and demons which inhabit the earth and cause impediments to good actions may remove themselves from here in the name of the order of Lord Shiva" and should forcibly hit the earth with his left heel thrice. 

Then he should chant the mantra that  I am starting the worship of Brahmam, without causing any inconvenience and without incurring the wrath of the spirits who hold this earth aloft and make it worth inhabitataion..

He should perform the japa fully understanding the meaning of the manthra fully and unequivocally.  He should sit in the seat of Darbha and wear the skin of black antelope and appropriate clothes during the chanting.  

It is clear that we should have the blessings of all our fellow human beings and gods if we have to start and complete a good action to its happy conclusion.  We can always expect the evil forces that will come in the way of good actions.  They are driven away by threat of the anger of Lord Shiva and also of physical force.  Besides some spirits are always present in nature protecting and keeping the world aloft.  Their natural movements and actions should not be disturbed by our actions, whether good or bad. So the intention not to cause any inconvenience to them is delclared here.  The wearing of Krishaajinam is restricted to brahmachaaris by convention now.