A few random tips I've picked up from organizing different things over the past couple of months:

Phones

• If you want to make sure your pre-paid DTAC SIM doesn't expire while you're out of the country for a long period, switch it to the "Simple 2 Baht" plan, and use your Siam Commercial Bank account's Internet banking service to recharge it. You can recharge from anywhere in the world, and you only have to recharge it with 50 baht to extend it's life by 12 months, so as long as you keep track of the expiry date, you can keep a prepaid card going indefinitely, and keep the same phone number active even while you're out of the country. Bangkok Bank doesn't offer a phone recharge service, but other banks may do.
• DTAC doesn't offer global roaming on pre-paid SIM's.

Banking

• All Thai online banking services are disturbingly rough looking. Bangkok Bank and SCB are ok. CitiBank is barely usable and needs to be rewritten.
• The annual fees on gold and platinum credit cards are cheaper in Thailand than in the West.
• Credit card rewards programs that give you Thai airways miles get you 1 mile per 20 baht, which is a better rate than in the west.
• Thailand's Internet banking services are still catching up in some areas: for example, you can't transfer money between banks electronically yet.
• Getting SWIFT access through SCB is way easier than getting it through Bangkok Bank. Less documentation and hassle all around.
• Bangkok Bank credit cards suck in that they freeze funds in your savings account equivalent to the credit limit of your card. Actually Bangkok Bank sucks in general.