I use one of the brand name concentrated fuel injector or fuel system cleaners every oil change - 3750 miles. I'll use STP or Gumout or Lucas, whchever is available at Wal-Mart while I'm grocery shopping. My 4 year old Mazda 6 Sport Wagon is still getting 29 MPG + on every tank.
Also try a tank or 2 of every major brand name. I don't know why but I always get the best mileage on Citgo 87. I've heard that some of the no name gas at convenience stores actually has more cleaners in it but I don't know anything about any of the stores in my area or the gas they sell to recommend trying any.
I use synthetic oil and only change it every 7.5k miles (govt studies on school buses recommend this) as well as trying to use higher quality gas (chevron, BP, Mobil, etc) and according to fuelly I have some of the best mileage of my car type. This may have to do with the type of driving that I am doing, but the past two months have been almost entirely city driving so I kinda doubt it.
When I changed my plugs last I didn't get platinum plugs as recommended by the manual and I think there was a 2-3 MPG diff but I don't have data to back that up, so take that with a grain of salt.
And lastly, as for the K & N brand filters, even if it doesn't add to the MPG you still never have to buy a filter again, since you jsut have to clean it and put it back.