It depends on how much power you are talking about.
One possibility would be to get an automotive relay, connect it to you ignition switch to control the power from the battery to your laptop.
So #85 to ignition switch power, #86 to ground (metal chassis ground)
#30 to your battery power, and #87 to the laptop power supply.
You can get relays mail order etc...but I was able to get one...with a nice wiring harness from my local car audio/ alarm dealer/installer.