My SE P910i, which i have owned for quite some time now, obviously uses a fair bit of juice, but mine can easily last from Friday morning until sunday evening without charging, with daily use.
It has a Li-Polymer battery, and i'm guessing isnt affected by 'memory' as i charge it every time i go to bed, even if it doesn't need it, i like to keep it topped up.
If you're having battery problems, then what you could do is purchase a new genuine battery (not some fake ebay job), and give it a full charge. Your phone will then be like new again.
I don't think they're that expensive either. Then with the old battery, if you find the +ve and -ve terminals, and connect them together with a resistor for a few days, this will clear the 'memory' it has accumilated. Once you've done that, give it a full charge, and hey ho you also have a spare!