Do you do a lot of short trips? If so, the battery may not be getting charged enough. I use a Battery Tender to top off the batteries in my vehicles if the voltage is a little low (12.6 V - fully charged, 11.9 V - fully discharged).
The Battery Tender is pricey, but still much cheaper than replacing a battery every year. A trickle charger could be used also. For example, Harbor Freight has this one for $9.99 regular price and is often on sale for 1/2 price.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=42292
My 2000 Blazer has driveability problems if the battery voltage is 12.2 volts or less (~50% charge).
Has the alternator output been checked? A weak alternator will kill a battery and vice versa.
Ed B