It does NOT sound right that your battery would die after 3 or 4 short trips. This instead sounds like an alternator with shorted diodes. Some mechanics do not know how to check an alternator for shorted diodes. Instead; they just check it for voltage output (which will usually be there regardless of whether or not there is a shorted diode) and then tell you the alternator is good, if it just produces voltage. The way to check for shorted diodes is; while the motor is off, all doors closed, key out, and no lights or electrical accessories on; disconnect the battery ground cable, and connect an ammeter in series between the battery negative post and the cable which was taken off that post. There should not be more than 50 milliamps of current flowing when tested this way. If there is significantly greater current flow than 50ma; there is a short in the electrical system; which probably is coming from a shorted alternator diode. If the alternator cable is then disconnected, and the drain stops; the alternator should be replaced.
Other mechanics only check the alternator voltage output at no load; but do not check the output under heavier loads. Here again; there are some defects which would still allow the alternator to produce 13.5 or more volts across the battery when no lights or accessories are running; but in which the battery voltage would drop substantially below 13.5 volts if the voltage was tested with the engine idling; while the lights, fan, rear window defroster, A/C, and emergency flashers were all running.
If you are willing to risk offending your mechanic by asking him to run these specific tests; it could be clearly determined whether the alternator is causing your problems. You could also avoid potential offense by taking the car to an auto electrical specialist, and having them run these tests.
Some brands of batteries are built to have short lives (really). Others will last more than twice as long. The longest lasting batteries are made by a company called Johnson Controls; but are sold under different names. Costco's Kirkland batteries are the best of these. Wal Mart's Everstart batteries are similar. And Auto Zone's Duralast is the third best. Sears batteries have a reputation for failing before it would normally be expected.
Trickle chargers are not intended to be used AFTER a battery dies. Their intended mode of use is to be connected when you park the car every night, during which they will keep the battery fresh and strong for the next morning's start. Trickle chargers (if you used the term correctly) are much too weak to recharge a dead battery in a reasonable length of time. You need at least a 6 amp charger to recharge a dead battery; and that would take at least 10 hours. (trickle chargers only put out about 1 or 1.5 amps). A 6 amp or 10 amp fully automatic charger is the best type to use; because automatic chargers can be left connected for long times and will not overcharge a battery; yet they also will charge a dead battery rapidly (if it is able to take a charge). But a well designed battery used with a strong alternator should not need to be charged under ordinary conditions; unless the temperature is frequently below freezing..
If the alternator needs to be replaced; please insist that the replacement alternator be purchased at a NAPA parts store. There is an epedemic of junk alternators being sold by popular parts stores; which often lead to endless confusion and frustration when the "rebuilt" part works no better than the bad one did. As a career electrical systems specialist; I found that I could only trust NAPA alternators.
I see no reason why your Subaru cannot give good service under your driving conditions; IF it is tested and maintained properly. But when I taught engine theory courses for aspiring mechanics; my students always said that electricity was the hardest part of the course for them to understand. So don't assume that an otherwise good and honest mechanic knows how to do proper electrical diagnosis.