I followed the base build order for my impossible play through in this link - http://www.reddit.com/r/Xcom/comments/11hi3w/what_are_some_recommended_build_orders_and/%29

It isn't possible to get the second month done at the right timing every time because of the randomness of when the abductions actually occur, but when it finally worked out for me I was only able to get 4 satellites in the first month, 3 in the second month (couldn't build the 4th satellite uplink in time because I didn't have enough engineers), lost 1 country in the second month and then covered the rest of the world in the third month.  From that point on you don't ever have to worry about panic again as long as you are succeeding at each mission.

A note about gameplay - consider using the outer limits of the maps to go around and try to attack from the back side of most of the maps, it will help out a lot and usually has far better cover.  Also, google how to utilize the flanking overwatch lock glitch, it helps a ton on impossible.