Well without modding too much nothing! I have the 328i and it does like to drink its fuel! i usually go through about £30 - £40 a week and thats not driving very far but I heard on the radio some tips:
1. Change gear AT 2,000 rpm
2. Try to not use 1st gear unless absolutely necessary (I mean the high torque engines are fine pulling in 2nd)
3. Never use the AC it subtracts about 5mpg
4. Try to not race up to traffic lights, if you see red lights just let your foot off the accelerator, try not stopping!
5. Don't go fast around corners!
6. Lose any unnecessary weight, if your like me then your boot is usaually full, empty it if possiable!
7. This is the best one... Let your engine warm up for 5 minutes before any drive. A car will run at like 50% mpg when the engines cold! (or something like that)
Hope this helps, Petrol Prices are a B****RD