Traveling off season.
I just did a week long trip to Switzerland and france for under $1000 including lodging, flight, and food.
Did another trip to england for $1100 flight/hotel (nice hotel).
Got a flight to Zurich a few years back for $200 round trip.
I typically find everything on Kayak and then go search flight coupons and stuff. You can typically get around $40 off the flight from Vayama. Sometimes expedia or orbitz will have a coupon code.
I also enjoy meeting up with someone and buying their flight vouchers from them (this can only be safely done in person to make sure the vouchers work. I was able to book LA to Minnesota and LA to CO for SIA/Xgames for $300 total and then sold remaining $150 worth of vouchers for $100.