What could be the problem?

I have a Chevrolet K1500 truck. It has the original 350 engine in it with a 150000 miles of it. I get about 11-12 miles per gallon with it right now and two years ago when I bought it, it got around 17 miles per gallon. Wondering if anyone would know why I am getting such bad gas mileage with it right now.

What year?
Extended idling, change in driving habits?
When was the last time you had a tune,,,what type of plugs are you running?

Start with the stuff gmmerlin said and if that doesn't work then you may have a cat plugging up, have the back pressure tested.