Battery problems

We have a 92 Pontiac Grand Prix LE wity 250,000km on it. We had to have the battery and alternator replaced and car ran fine for awhile then it just would not start one day. Told it was a bad battery and another new one put in. After that the service engine soon light would come on and the battery light would flicker on and off, other lights would dim-took it back to mechanic -told I had a short and it was supposedly fixed but now unless you start the car every day it has to be bbosted to get it going. If it sits for one day the following morning it will not go. We were told to just start car every day because mechanic did not know what was wrong. Obviously something is draining the battery. Any help would be gratefully appreciated. Thanks, Connie