These are specific to the recitals one is performing. No such general rules are there except possibly the tradition of chanting prayers to one's Guru and Lord Ganesha using slokas or mantras.
When one is reciting the stotra of a particular deity then at the beginning the Dhyanam of that particular deity is recited.
For example, at the start of reciting the Chandra Kavacha Stotra one recites his Dhyanam:
Asya Sri Chandra Kavacha stotra Maha Manthrasya
What follows is the great prayer called “Armour of the moon.”
Gowthama Rishi Anushtup Chanda, Chandro Devatha Chandra preethyartham
Sage for this prayer is Gowthama, meter is Anushtup, God is the moon
and this is being chanted to please the moon.
Samam, chathurbhujam vande, keyura makutojjwalam,
Vasudevasya nayanam, Shankarasya cha bhooshanam., 1
I salute the four armed Moon god who is calm, Who shines in a gem
studded great crown, Who is the eye of Lord Vishnu, And the ornament
of Lord Shiva.
Yevam dhyathwa japen nithyam shasina kavacham shubham,
shiro desam, phalam pathu kala nidhi., 2
Thus praying,this holy armour of moon, should be chanted daily, Let my
head be protected by moon, and hairs be protected by the treasure of
The verses that are in the bold form the short Dhyanam of Chandra for this particular stotra. Same is the case with any other stotra or mantra.
For Rahu Kavacham:
Pranamami sada rahum, soorpakaram kireetinam,
Saimhikeyam karalasyam, lokanamabhaya pradham., 1
I always salute Rahu, who holds a winnow, who has a crown, Who is like
a lion and is terrible looking, And who grants protection to people
Neelambara sira pathu, lalatam loka vanditha,
Chakshushi pathu may
Rahu, srothre thwardha sareeravan., 2
Let my head be protected by he who wears blue silk, Let my forehead be
protected by he who is worshipped by the world, Let Rahu protect my
eyes and semi bodied one protect my ears.
The bold portion above is Rahu's Dhyanam which has to be chanted at the start of the recital.
Apart from this, before staring any Puja or ritual usually one prays to one's Guru using mantras like Om Gum Gurubhyo Namaha or by using Slokas like "Gurur Brahma Gurur Vishnuh .. ". And then one prays to Lord Ganesha using mantras like Om gam Ganeshaya namaha or slokas like "Vakratunda Mahakaya ...".