GoogleFlights allows you to track prices for the date/routes you are interested in and will send you an alert if a significant from normal price change has occurred (higher or lower).
Best advise is when you find something in your price budget, book it and then monitor for flight changes. While each airline is different, when the airline makes a schedule change, it generally opens up other options for you to look at with better timing, fewer stopovers (generally the most expensive flights are direct) etc
We always book direct with the airline vs using third parties.
Interesting that most on these boards are happy to use a TA for cruise booking but choose to book flights, hotel etc directly.