Water Heaters - Hard Water

I replaced our hot water heater yesterday after only 6.5 years of service. It was an electric Bradford White water heater. For the past couple of months the water has been getting cooler as you take a shower. Two nights ago we did not have any hot water. I found the reset button had tripped, so I reset it and the water heater began to run. The very next day the water was cool again so I checked the reset button and it had tripped again. I then checked both heating elements and noticed the bottom one was allowing a drip of water about every 20 seconds, which dripped on the ground wire. I figured the bottom element was bad, so I took it out (after a lot of twisting) and it was covered with lime...large white chunks. Once I finally got it out, I noticed the whole bottom of the tank was full of these white lime pieces. After about an hour of trying to clean it out I dicided we would be better off just replacing the tank. The bad thing is with the hard water do I just expect to replace it every 6-7 years or is there a way to offer it some protection. I don't want to put in a water softner in to many issues with location and such, but if I can protect the water heater I would like too. Any suggestions would be appreciated.

You got the idea: water softeners extend the life of water heaters.

Is there any type of filter a person can place on the cold water source line prior to entering the hot water heater?