If you want to increase power and mileage (even if just a little), then go with a free-flowing air intake filter replacement from, say, K&N. Perhaps a set of replacement spark plugs will help. From what I've heard, the best parts for Toyotas are made by Denso.
Really, if your 'Rolla is running well, I'd just concentrate on using the best air filters, best spark plugs, best oil filter, and best oil. Always keep your tires inflated, and come to a stop slowly and leave off from a stop slowly for maximum MPG.
If you'd like to trick your ride a bit, a TRD (Eibach) spring set with matching hardware and a sleek set of rims will work wonders. Check out these pics for yourself:
Lowered Corollas
Cheers