Whenever a good deal comes up to somewhere that seems cool, I just book it. There is a chance I won't be able to make the trip work, but in the long term, the cost of periodically missing a $300 round trip to Europe or a $500 round trip to Asia is far lower than always paying $1000, and for me the spontaneity of going to new and exciting places sometimes last minute is absolutely worth it.
This can be very difficult to coordinate with my job, especially when the deals come up last minute, but we usually find a way to make it work.
Sometimes friends and I will want to go somewhere specific. Our usual strategy in this case is to not book anything because always one of us cannot 100% commit, or one of us has a hope in the back of our minds that a deal might come up. We almost always end up committing in the end, and a deal almost never comes up, so we normally end up paying the highest prices at the last minute. Though we do get lucky sometimes :)
As for choosing destinations: we're an outdoorsy friend group, so the combination of all of us reading nature and travel blogs / magazines, following Instagram feeds, etc. is enough to give us a bucket list. Usually travelling inspires me to visit more places. Whenever there's a really good deal on YVR Deals, I'll use that as inspiration to go somewhere new.