I'll be on the road for upwards of 9 months in Asia and plan to use my ATM card (knowing that in some countries like Cambodia, they don't have ATMs), and will also use credit cards on occasion for air fare, etc... I'm also planning on taking travler's checks for those ATM-less countries, but also for extra peace of mind and to avoid the necessity of using ATMs ALL the time (my bank charges a $3 fee for withdrawls in foreign countries).

I just have no idea how much cash to put towards traveler's checks for this length of time, and what denominations. Anyone have a good (simple) forumula for deciding what to take?

Small numbers of travellers checks. Really, 200 bucks US in those countries will get you through a long time, until you have access to another one. Wherever you can cash TC, you will be able to reach an ATM, and that exchange fee is still usually worth it for larger amounts (200-300 US) because of the comission charged on TC.

I like carrying about 1000 dollars worth just for emergencies, and have used them for such emergencies many times. It's NICE to have refundable money if you are robbed. Its nice to have enough to go home or pay for emergency medical aid.

Course, 1000 dollars may be overkill. Enough for a return ticket may be sufficient in most cases.