If money wasn't an issue, I'd only fly jetblue direct, Boston - Vegas.
Alas, money is an issue, so I'm booked on Delta, with a stopover in Cincinnati on the way out and Atlanta on the way back. I must admit that i have a soft spot for Delta since they put me in first class once, for no good reason that I know of. It was nice, and I tried to act like I had been there before.
When I have the dates I want more or less, I use kayak.com and bing.com/travel to check prices. Both are super-easy to use and both show calenders with prices, which are nice when your dates are flexible. Both will also let you set up an alert so they can send you an email with fares.
Airline fares change all the time, when the fare you want comes up, you have to be ready to grab it. Other people have had good luck with deals on facebook and twitter, but those haven't worked for me yet.
It's always cheaper to travel to Vegas during the week than the weekend - Sunday, Monday & Tuesday are the cheapest days to fly.
When booking my flight, I believe in the old adage that the airlines release fare deals on Tuesdays and Wednesdays, so these are the best days to book. Don't book your flight on a weekend, fares are highest because that's when the most people are booking. I book direct through the airline websites, which means I spend a good deal of time going from one to the next to find the right price. I also don't mind flying the redeye, so long as I can get back to Boston while the subway is still running. Arriving late night in Vegas is no problem. Vegas never sleeps.
Anyway it took me some time and some searching, but I found the fare I wanted - 255 round trip, on Delta. I also took a big gamble - bigger than any I'll take in Vegas - a 41 minute changeover in Cincinnati. So if you see me running, please be kind and get the hell out of my way.