What are the general guidelines and halacha l’maseh when it comes to making the bracha “HaTov V’haMativ” on wine.
In brief, there are two conditions for making the berachah. The first relates to the relative quality of the wines.
Although this involves a dipute among rishonim, the ruling of the Beis Yosef is that one recites the berachah of hatov vehameitiv whenever a second wine drunk following a first session of drinking wine, provided that the second wine is not clearly of inferior quality to the first.
When the second wine is white wine, and the first wine red, the berachah can always be made, because white wine is deemed healthier.
The second condition for the berachah is that the wine should be drunk in a group, meaning at least by two people.
The Rema adds that where both wines were present from the beginning of the meal, the berachah is not recited.
The Magen Avraham cites a ruling whereby one should not make this blessing if one has finished the first wine that he had. Some express doubt concerning this halachah.
An important note is that if a person is presented with two wines, a superior wine and an inferior wine at the start of the meal, one should not drink the inferior wine first just so that he can then say the berachah. Rather, the concept of chaviv is invoked, meaning that the berachah of ha-gafen should be made on the superior wine.
See Orach Chaim 175 and commentaries.