I have a price in mind that I'm willing to pay and that I know is a reasonable fare based on history. From Houston, I'm looking for $99 each way + taxes, so around $220-$230. If a flight at that price (or lower) is available when they extend dates, I'll go ahead and book it. I'll then keep checking, and if there's a lower fare, I'll rebook because I know we'll be flying again and will use the credit within a year. If only higher priced flights are available, I'll wait for a DING or other special.
This is only when I've planned my travel around non-peak times. If I'm flying at peak Thanksgiving or XMAS times, I don't expect to get a $220 fare. I tracked the days last year when good fares were available for our upcoming Thanksgiving trip, and I also avoided travel on days that are blocked out for Southwest's Rapid Reward award tickets. In general, the blocked out days are days where they won't have the lowest fares anyway.