Plumbing problem!! I had a plumber come out about 6 months ago to clear my pipes... I'm guessed right, it was a backup problem. The plumber told me the tree roots were clogging the main pipe and it would take a "snake" to clear it. Plumbing liquid down the tub won't help this case. In fact, he said it would eat away my pipes. It's now seeming to be the same case again. How do I prevent this backup problem without having to call a plumber every 6 months?? :confused:

First get the roots snaked out . Going in, let me say that sooner or later you're going to have to bite the bullet and replace that old sewer line with PVC or repair the break where the roots enter. As for root control RootX or Robics Foaming Root Killer that contain Dichlobrnic are two products you could try.
A less expensive way would be to call around to garden supply stores and ask for fine grain Copper Sulphate. Put a 1/2 pound in your toilet and flush it down. Repeat in 6 months.
Hope this helps Tom