Tips to improve fuel economy:
Tires. Properly inflated tires can improve gas mileage. Subaru is generally 33-35psi, but check your tire sidewall or the manufacturer sticker on the door jam. Higher tire pressure, gives a slightly firmer ride and better economy.
Avoid jackrabbit starts. Zoom-zoom is fun but bad for gas economy.
Cold engine. A cold engine will not accelerate as well, as quickly or as smoothly, as a warm one. Don't push it.
Empty extra weight and junk from the trunk.
Use the recommended motor oil. Subaru uses 5-30w.
Roof racks. Remove things from the rack that cause air drag.
Avoid fast-slow, speed-up and slow-down, on and off the gas pedal. Steady is better.
Air conditioning, turn off when not needed. Note that A/C dries out the air and is needed to defrost the car and windshield, and most cars don't give you a choice but if they do, use the a/c for safety. Also the Mythbuster guys (on TV) did a test and found that a/c uses less gas than driving with windows down. And a/c should at least be used monthly to maintain the system.
Avoid excessive idling, but it's good to let any cold engine run for 15-20 seconds or so before putting it in gear.
Try not run out of gas and suck in debris and gunk from the bottom of the tank that clogs the fuel system. Condensation and water can form in an empty tank.
Clean air filter. It's easy and makes a difference.

New for 2008 Model Revised EPA Fuel economy testing methods will result in lower but more realistic ratings for all manufacturers. New lower estimates (starting with 2008 models) more accurately reflect our current stop-and-go traffic and commute driving patterns, faster speeds and acceleration, air conditioner use, and colder outside temperatures. MPG estimates will also be adjusted downward to account for factors that are difficult to replicate in a laboratory, such as wind and road surface resistance. New fuel economy estimates will be lower for most vehicles. This is not because auto makers have designed the same vehicles to be less fuel efficient—it is because our new test methods take into account factors that have been missing or not fully accounted for in the current (2007 model)  tests. Because some vehicles are more sensitive to these factors than others, the impact of the proposed changes will vary from vehicle to vehicle. The city MPG estimates for conventional (non-hybrid) vehicles would drop on average by about 10-20 percent from (2007 model) labels, while the highway estimates would drop by about 5-15 percent, depending on the vehicle.  Hybrid vehicles, city MPG estimates would drop 20-30 percent from (2007 model) labels. highway MPG estimates, the change is about the same as for conventional vehicles. More from the EPA
 

What it costs to drive that car each year.  Fuel cost based on 15,000 miles per year (1,250 miles per month), price per U.S. gallon in U.S. Dollars What 1 gallon of regular gas costs : 1970 $.34, 1980 $1.24, 1985 $1.15, August 1990 $1.20; August 1995 $1.12; August 2000 $1.43; March 2005 $2.03; April 2007 $3.25, April 2008 $3.89
Gallons used driving 15,000 miles per year what many people drive per year, or 1,250 miles per month	price per gallon   ---     price per gallon     ---   price per gallon      ---           price per gallon      ---          ice per gallon
	$4.00	$3.80	$3.60	$3.40	$3.30	$3.20	3.00	$2.80	$2.60	$2.40	$2.20	$2.00	$1.80	$1.60	$1.40	$1.20
At 70 mpg you used 214 gallons driving 15,000 miles per year and it cost this much (divide by 12 for what it costs monthly):	$857	814	$771	$731	$710	$685	$645	$600	$557	$514	$471.	$429	$386	$343	$300	$257
48 mpg (uses 312 US gallons to go 15,000 miles)	$1248	1185	1123	1061	1030	998	936	874	811	749	686	624	562	499	437	374
45 mpg (uses 333 US gallons)	$1332	1265	1198	1132	1098	1065	999	932	866	799	733	666	599	533	466	399
41 mpg (uses 366 US gallons)	$1464	1391	1318	1244	1207	1171	1098	1024	952	878	805	732	659	585	512	439
38 mpg (uses 395 US gallons)	$1580	1501	1422	1343	1303	1264	1185	1106	1027	948	869	790	711	632	553	474
35 mpg (uses 429gals)	$1716	1630	1544	1459	1416	1372	1287	$1200	1114	1029	943	857	771	686	600	514
33 mpg (uses 455gals)	$1820	1729	1638	1547	1501	1456	1365	1273	1182	1091	1000	909	818	727	636	545
31 mpg (uses 484gals)	$1936	1839	1742	1645	1597	1548	1452	1355	1258	1161	1065	968	871	774	677	581
29 mpg (uses 518gals)	$2072	1968	1865	1761	1709	1657	1554	1448	1345	1241	1138	1034	931	828	724	621
27 mpg (uses 556gals)	$2224	2113	2002	1890	1834	1779	1668	1556	1444	1333	122	1111	1000	889	778	667
25 mpg (uses 600gals)	$2400	2280	2160	2040	1980	1920	1800	1680	1560	1440	1320	1200	1080	960	840	720
23 mpg (uses 653gals)	$2612	2481	2351	2220	2154	2089	1959	1826	1696	1565	1435	1304	1174	1043	913	783
21 mpg (uses 715gals)	$2860	2717	2574	2431	2359	2288	2145	2000	1857	1714	1571	1429	1286	1143	1000	857
18 mpg (uses 834gals)	$3336	3169	3002	2835	2752	2668	2502	2333	2167	2000	1833	1667	1500	1333	1167	1000
15 mpg (uses 1000gals)	$4000	3800	3600	3400	3300	3200	3000	2800	2600	2400	2200	2000	1800	1600	1400	1200
12 mpg (uses 1250gals)	$5000	4750	4500	4250	4125	4000	2750	3500	3250	3000	2750	2500	2250	2000	1750	1500
8 mpg (uses 1875gals)	$7500	7125	6750	6375	6187	6000	5625	5250	4875	4500	4125	3750	3375	3000	2625	2250

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this is all unofficial and subject to change, correction etc
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