Here’s an handy trick that will help you to refine your Gmail searches.
Sometimes i found myself searching for key words in conversations with one of my contacts. That conversations can be either chats or a standard e-mail interactions.

In case i am searching for the keyword “proposal” only in chat conversations i had with John Doe i’ll use the following command:

In case i am searching for the keyword “proposal” only in e-mail conversations i had with John Doe i’ll use the following command:

Note: Alternatively i can use the +label:chat and -label:chat commands.