Some gmail advanced search tips which can be useful if you are heavy gmail user.
- File all unread messages in inbox
- Find a thread which some keyword where you have involment
This is pretty handy to find some thread where you vaguely remember the subject.
- The thread has attachment
- Finding big emails (e.g. email of size > 1M)
These are some of the tips I use often. There are many more tips at Gmail help official page in case you want to read more.