I had one done for free on a duplex I lived in up in Columbia, MO. I knew most of the stuff that he told me but I did learn a few new things. I put most of the stuff I already knew plus what he told me to use in the house we bought (between Columbia and Ashland). I thought I did pretty well but still thought we were loosing more heat/ac than we should be.
I just had another one done in Dec/Jan. I knew we needed more insulation in the attic so I found a company that would do something called a "blow test". It tests the airflow through your house. They did the test for free (normally around $400) provided I agreed to purchase insulation from them. I knew I was going to do that anyway. They did one test before they put in insulation and one test after. Turns out my house did need more insulation, which I already knew, but it didn't really need anything else. The person that did it told me after the initial test that our house has an airflow VERY similar to that of a new house. (Our house was built in the 80s.)
Our electric company also give rebates based on the change in air flow after you do what ever improvements are suggested. We will get back just less than half of what we paid for the insulation.
Like others have said, even the smallest crack/holes can make a huge difference, this includes your outlets and switches. You can buy insulation pads that simply slip behind the covers. You can also buy other simple to use items to help fill in cracks and holes. A bonus to filling in the cracks and holes is that it will help keep out mice and bugs. If you have a mouse problem I also suggest that you use some steel wool to help fill in the holes and cracks. Mice won't chew through steel wool because it cuts up their mouth.
If you would like more information or the name of the company I used for the insulation please send me a Private Message. (The company is VERY professional, prompt and know what they are doing. I am not 100% sure but I think they come down the Jeff. City too.)
L.
P.S. Another thing that I was told was that cobwebs are actually a good thing. :) They tell you where you have airflow in the house because spiders build their webs near airflow. To a spider, airflow normally means flying food.