Of everything you've done, slowing down will probably have the biggest positive impact on fuel economy. Several of the other items will have no to negligible effect.
In general, changes to your driving habits will greatly exceed the impact of mechanical mods. And changing driving habits is free.
The best money to be spent in the beginning is on fuel consumption instrumentation like a ScanGaugeII -
http://scangauge.com if your car doesn't already have a stock display.
PS -welcome to the site!