What to chant before taking and consuming prasad from anyone for any god ... is there a common mantra or thank you shloka ?
As far as I'm aware of there is no such Mantra/Sloka which should be chanted while consuming Prasada. But there is a Sloka which is usually chanted while consuming Tirtha (or holy water which has touched the deity). It is also chanted by the priests in many temples while distributing Tirtha water to devotees.
What the devotees do is accept the Tirtha in their right palm, chant the Sloka and then sip the Tirtha. This is done with the belief that by consuming this holy water one can avoid untimely death, diseases etc.
Quoting from Hindupedia.com, the following is the Sloka:
45.To take the sacred water after anointing God
1.Akala mruthyu haranam , sarva vyadhi nivaranam,
SAmastha papa kshaya karam , Shiva(Vishnu) padodakam ||
The water washing the feet of Lord Shiva(vishnu) Would avoid accidental early deaths , Would cure all diseases, And would also destroy all the sins committed.
Also, this Sloka is a general one. One can replace Shiva/Vishnu in it with the deity of which the Tirtha is being consumed.
Also, note that between Tirtha of God and his Prasada there isn't any difference. Both have equal grace and touch of God. So, if one wants to modify the Sloka to Prasada taking scenario, one can simply replace the word "Padodakam" in the Sloka with "Prasadam". Or else one can simply chant the same Sloka as it is.