The Rishonim (Early authorities) discuss what to do if one took a sip of a beverage and before swallowing, remembers that he forgot to make a Beracha. They concur that he may not make a Beracha with his mouth full, yet they allow him to swallow the beverage, as spitting it out would ruin it.
The Poskim disagree as to what he should do after he swallows it. According to Rashi and Rabbenu Hannanel, after he swallows it, there is nothing more to do; he has missed the Beracha. On the other hand, the Rosh holds that he can still make a Beracha of Shehakol on the swallowed beverage retroactively. This is a rare case, in which there is an opinion that one can make a Beracha on a food "after the fact."
The Shulhan Aruch rules in accordance with Rashi and Rabbenu Hannanel that one does not make a Beracha after he swallows. The Rema rules in accordance with the Rosh. The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) points out that the accepted practice of Ashkenazim is to follow Maran, and not the Rema.
Hacham Ovadia presents a Chidush (novel approach) as to what to do when the beverage is still in his mouth. He rules one may "think" the Beracha, without verbalizing it. Although, in general, "Hirhur" (thinking) is not tantamount to speaking, in this case there is no choice. Hacham Ovadia goes so far as to say that, based on that Beracha, he is allowed to continue drinking more beverages, without verbally articulating an additional Beracha.
SUMMARY
If one forgot to say a Beracha on a beverage, he cannot verbally recite it, while the liquid is in his mouth. He may "think" the words of the Beracha, and that would not only exempt the liquid in his mouth, but it would also cover any additional beverages he wants to drink.
