First bit of advice: get a ScanGauge or equivalent. If you're serious about MPG, it will pay for itself many times over. It would have showed you the horrible direction of that first tank well before fill-up time!
I trust you've seen our tips:
65+ Efficiency Mods
100+ Hypermiling Tips
Aside from basic mods like tire pressure & possibly partial grille blocking, I would start with the driving techniques. Make it a game: every commute is a challenge to beat your previous high score (need the MPG gauge to play this game, obviously).
Should have got the manual though! Much more fuel economy potential than with an automatic.
Welcome to the forum!