We all seem to agree on the basic facts but have different approaches to handling them.

First, exchange rates into baht are horrible outside Thailand. Avoid changing abroad at all costs. Arrange an airport transfer through your hotel so you don't have to worry about it upon arrival, or get a small amount of baht at the airport, which has multiple booths and ATM's.

The best rates are on high-value t-cheques, the $100's. Lower value have worse rates or higher fees. T-cheques get better rates at most booths and banks than banknotes, but it's not much difference, fractions of a baht.

For exchanging cash or t-cheques, many banks and booths require a passport but will often accept a photocopy.

Avoid changing at the front desk of a hotel in all but an extreme emergency. I have seen them offer rates about 10% lower than bank rates.

The major commercial banks have ATM's throughout the city that provide smooth exchange. Those include Bangkok Bank, Siam Commercial Bank, Thai Farmers Bank, Thai Military Bank, Krung Thai Bank. Learn to recognize the logos of the banks (doesn't matter which is which, as long as you know which ones are the major banks) so you can spot a bank at a distance if you're in need in someplace like Chinatown, where banks can be hard to find. Some of the smaller banks don't do exchange or might not have ATM's with English displays. If you're not with a Thai, bank workers can help you get to the English screens if necessary.

You will need cash for many transactions in Thailand, such as tailors, fabric stalls, and local goods merchants. Don't expect to use your American Express very much at Chatuchak Weekend Market, though some stalls will accept cards.

My own practice is to have a small amount of baht in hand upon arrival from the last trip (if possible), bring a couple of hundred dollars in US cash (which you'll most likely use upon return to the US for the cab ride home), and bring a pretty good quantity of t-cheques. If you're going to have a major transaction in cash, such as a thousand dollars or more to pay your tailor, it's just easier to change t-cheques than run to the ATM all the time two or three times a day, and the fear of the ATM denying you is enough to give you pause. For ordinary expenses, I just use the commercial bank ATM's, taking about 10,000 baht at a time. If you have lots of t-cheques left over when you come home-- no problem, just change them back to cash (usually no fee) and put it in your bank account.

Granted, if you're getting hassled for 1-2% fees on each cheque when you buy them, that changes the equation. Flashing an Amex card at the Amex Travel Services window solves that problem, though.