I have always been religious about backing up my data, but was tired of pulling out my portable drive and connecting it whenever I wanted to back something up. This isn't the most elegant solution by any means, but it doesn't add much weight to my system and I can always backup and have access to other files if I need them.
1. Get yourself a slim USB drive that doesn't need any power. I use the Western Digital. They are small and go up to 320GB.
2. Add some velcro to the back of the drive and then to the back of your laptop. Ideally skew it to the side of the computer that has the USB/Firewire ports so you can use the short cable that comes with the drive.
3. Velcro on and go.
This would also work with a USB powered USB hub.
Step 1: Attach Velcro
Just attach two pieces of velcro to your drive and laptop.
Step 2: Attach the Drive to the Back of the Laptop
Just place your velcro in position where there is enough room for your lid to open.