I would think about switching away from the stock 17" runflat tires setup to something lighter. The stock rims are quite heavy as are the stock runflat tires.
Cosmetically, 18" or 19" rims will look nicer, but you want to opt for a well constructed lighter 17" rim which is at least sub 18 or 19 pounds. You can get energy efficient tires which have less rolling resistance which will help. But usually their performance will be limited. Something narrow would probably better efficiency wise. So skip a staggered setup.
All this will reduce your unsprung weight and rolling resistance which will ultimately will help in mileage. However only expect a 1 or 2 mpg increase. Your investment will cost you about $1000 to $2500. So the cost benefit ratio for this mod is quite low and you will need to drive a lot to recoup your costs.
A well constructed intake may help. When I had my 325i people were saying the Gruppe M replica intake gave a couple mpg increase. If purchased on ebay it will cost about $400.
Your biggest increase in mpg will come from driving with care and a light foot over the gas. That alone can make a difference between 27 mpg on the highway and 36 mpg (doable on a 325i).
Do not coast in neutral as that uses more fuel than if you are in gear coasting. If you drive stick try to keep in gear as long as possible when coming up to a light.
Keep all your tires well inflated. Higher psi will also reduce rolling resistance. 36 psi or above would be best. Check the rating on the tires. 40psi might be ideal for gas efficiency but you may feel the ride a little harsh.
Do frequent engine oil changes. I would recommend doing them more frequently than is recommended by BMW as I've noticed my mpg go down when the oil is old. Keep the engine serviced...you may want to talk to a independent garage recommend some preventative maintenance such as coolant flush and tranny/diff flush (even tho BMW says it is lifetime fluid it might still be advisable)