When does airfare decrease for spring travel?
 

Trying to book a trip in early March for a dull, non-vacation city in the U.S. (San Antonio). The fare seems quite high, mid 400's to 600. I think I'll wait. Maybe it's cheaper in January? If not, I'll bite the bullet and buy in late January.

All depends upon supply and demand and no one can forecast that with any certainty. I disagree, strongly, that SAT is a "non-vacation city". Plus, with a hub airline, that is almost not a factor anyway. I agree that $600 seems high and it might come down but it might not either.

Your origination also is a factor. Are you flying from a small college town at the start of spring break?

While you can't predict them, sites like www.farecast.com can maybe help.

Finding the best fare
 

For a given city combination there is usually a minimum fare the Delta is willing to sell for. This minimum fare tends to have more availability during soft travel times which begin after new year and continues through the first or second week of March. I would do a multi day search on one way fares picking a Wednesday of each week through February and March with 3 days before and after. This will given you after scanning the whole list this lowest price for your travel period and the lowest cost for the various days of the week.

Use this lowest price to use as a price reference to determine your travel dates for lowest possible prices and relative value of current price for the flights you prefer.