Maintaining heat in large terrariums

[Deven]

Hey everybody, I'm going to be building my terrarium soon and was wondering what others are doing to keep larger terrariums within the proper temperatures. I'm planing on building it 4ft tall and 4 ft wide and I'm still debating on 6 or 8ft long.
I'm sure I'll be able to keep the basking side at the proper temps, but how can I keep the low end at the proper temp.?
A friend of mine recommended making a second basking spot but I'd like to get more opinions. Thanks for any and all the help.

 

[Walter1]

Maintain a gradient of suitable basking temps at one end grading to mid-upper70s at other. They can choose to bask, be active warm, or rest in relief of cool. If cool end is cooler than you want, provide another light.

 

[Deven]

That's Walter. I've never had to maintain a terrarium of this size and I don't want to mess up.