You'll want to lug Lenmar Enterprises' $159.99 ChugPlug with you when you travel. It's a portable power pack and battery charger for the MacBook Air and 13-inch MacBook Pro that provides up to four extra hours of juice for the former and three for the latter.
The ChugPlug is the same height and weight of Apple’s 60-watt MagSafe and connects with the MacBook power brick. Previous portable battery/charging solutions such as the Sanho HyperJuice meant you had to do a bit of hacking on Apple's power plug. Not with this baby; the Lenmar gadget plugs into your laptop's power adapter. Your MacBook's power brick attaches to the right side of the Chug Plug. It charges the external battery pack even as you use your laptop.
The ChugPlug has over-charge protection, but you should unplug it once it's fully juiced up. Leaving it, or any battery plugged in after it has completely charged, can negatively effect its ability to hold a charge.
The Chug Plug's orange LED illuminates when it's on or charging. The remaining three green LEDs indicate the charging status when it's charging. They also show how much juice remains in the ChugPlug. Conveniently, you can also check the remaining charge by quickly pressing down the power button.
At 9 x 3 x 1.3 inches and 1.1 pounds, the ChugPlug is a bit hefty. However, considering the extra power it provides your Mac laptop, that's a small (no pun intended) price to pay.
My one complaint: there's no version for the 15-inch MacBook Pro.