Best search engine for airfares:

 Kayak.com is my favourite online search engine for airfares.  I like to use their "Make my date flexible" option.  There are three options:

1) +/- 3 days: If I have a specific time that I want to book my tickets, this would be the perfect option.  It gives you a chart of the cheapest deal like:

Fares shown are the best available matching your filter selections.  

Return → ↓ Depart	Fri 10/1	Sat 11/1	Sun 12/1	Mon 13/1	Tue 14/1	Wed 15/1	Thu 16/1
Wed 4/12	$1387	$1387	$1467	$1387	$1217	$1217	$1102
Thu 5/12	$1404	$1404	$1484	$1404	$1234	$1234	$1119
Fri 6/12	$1404	$1404	$1484	$1404	$1234	$1234	$1119
Sat 7/12	$1404	$1404	$1484	$1404	$1234	$1234	$1119
Sun 8/12	$1387	$1387	$1467	$1387	$1217	$1217	$1102
Mon 9/12	$1387	$1387	$1467	$1387	$1217	$1217	$1102
Tue 10/12	$1387	$1387	$1467	$1387	$1217	$1217	$1102

2). Weekends: flexibility of cheap fares around the weekends

3). Flex month:  My favourite option that I can view when would be the lowest season throughout the month. 

*Kayak is also good about setting up the Price Alert.  When I want to book a flight ticket on a specific time, it will send me email (daily or weekly) on the prices.  So once it dropped, I can book the tickets right away.  

It also has Price trend feature:

Price Trend

Advice: Wait

Confidence: 61%

Prices may fall within 7 days

Prices may fall within 7 days

61% Confidence: Our model has been 61% accurate on forecasting whether fares for flights between these cities will fall $21 or more from the current price over the next 7 days. The forecast is based on analysis of historical price changes and is not a guarantee of future results.

or

Price Trend

Advice: Buy

Confidence: 70%

Prices may rise within 7 days

It has no guarantee on the price will drop, but I found it quite accurate and useful so far.  

BUT: I don't use kayak.com to book my tickets, because I read a lot of negative reviews on this web tool.  It is an awesome search engine when you have the flexibility.  It covers most of the airlines available, yet sometimes I found some of their fares don't exist on other websites.  Sometimes when I compare results from kayak.com and other websites, the flight number is the same but the schedule times are different from the original airline site.  Quite a number of people complained about their poor customer service and that they would charge their credit cards, but sent back an email saying the deals were no longer valid.  

So I usually book my tickets through Expedia.ca:

I had been booking through expedia.ca for years and I have no problem with their booking system.  I booked my hotels and flights through expedia and my credit card actually gives me extra points from booking through there.  That makes expedia my top choice to book my hotels and airfares from.

Hope my tips are useful :)