Generally speaking, car additives simply take money out of your pocket and into the sellers pocket. The only auto additive I've EVER used is octane booster on my `65 Corvair, won't have to do that any more with the new pistons used when we rebuilt it.
The old saying "If it sounds too good to be true, it probably is" rings true on many things, especially on car additives, magnets, etc. Save your money for more important things.
Driving habits will affect your FE more than anything else....learn to anticipate stoplights. as stated above, neutral should become an automatic reflex. I've been really working on this, and am doing much better on FE. I do a "dead stick" (power off) coast the last 1/2 mile going home, always try to keep my RPM between 1-2K when under power.