Sleep is non-negotiable. It's different having to get up every morning and still be functional. You can't overdo things and crash; you've got to actually consistently be functional. For some reason, I seem to need more sleep when on the road - 5 hours, maybe. It's weird. Maybe my body doesn't rest as deeply in strange random beds? Maybe timezone changes actually do affect me?
Transit time is not necessarily rest time.
Travel light. If it's more trouble to haul it than to buy it if you need it, don't take it.
Not having access to normal office utilities like printers, copiers, and fax machines adds up in terms of time, hassle, and sometimes even cost. And wifi. I'm seriously thinking about investing in 3G if I keep up this time on the road and the road continues to be in the US, because the $N per month I'd pay to have it vs the X hours per month I'd spend searching for internet or losing productivity due to lack of internet (or to get it in my hotel room when it isn't free) is probably worth the tradeoff. 'Course, if I spend a long stint not in the US, things get complicated.
Expense reports can be the bane of your existence. Doing receipts right after you get each one is unwieldy. Keeping them in your wallet is suboptimal. If you keep an envelope for reimbursible receipts (in chronological order) into which you toss things (with ballpoint pen annotations) right as you get them, then... well, we'll see how that one works out now.
What else is there? Road warrior travel tips?