This greatly depends on the storage capacity you need.
What I do is that I partition the hard drive on my home computer into one for the OS/Applications and then another called Client Files (let's say).
Then on the MacBook Pro I do the same thing, two partitions, one for the OS/Applications and then another called Client Files.
Then, when I'm ready to travel, I connect the MacBook Pro to the desktop computer via a firewire cable and start it in the target Disk Mode (hold down "T" at startup on the MBP and it will show up as a hard drive your iMac's desktop.
Then I use a program like SuperDuper to sync the two partitions and copy all info from the iMac *TO* the MBP.
After I go and travel, I simply use SuperDuper to sync the files *FROM* the MBP to the iMac. Thus, all client files are always kept up to date and in sync.
Again, this only works if you have a large capacity on your MBP, etc.
Another option is to simply lug the client files as an external HD on your travels, while having a backup of all of those files at home as well.