Note: Some of the points are Siem Reap-specific.
1) Tell the banks
A quick call is sufficient to inform the bank(s) that you're traveling abroad. This is to ensure you can use the card(s) for ATM withdrawal/credit card payment. I failed to perform this step before leaving; luckily the hotel manager was kind enough to let me use his phone to contact the banks.
Tip: it is good to know whether the country you're traveling to has your bank ATM. Else check the merchant logos on your debit card (eg VISA Plus). However do check with the bank on charges you may incur upon withdrawal. Eg Maybank charges RM12 per withdrawal (regardless of the amount withdrawn).
I always make a spreadsheet to record my expenses after each trip. Despite going to Siem Reap twice, I failed to check my spreadsheet and misjudged the amount of cash to carry. As a result, I'd to withdraw cash from the ATM here (luckily the exchange rate wasn't that high) and pay the hotel bill with credit card.
Edit: I did not change any money for my Bangkok trip thinking that the exchange rate there would be better; boy was I wrong!
Tip: check what transactions can be covered by credit cards. Typically hotels and better restaurants will accept credit card; however in Pub Street, many bars and restaurant only accept cash.
Spend some time to browse for deals/offers in the country you're traveling to. Case in point: I wanted to watch the Apsara performance (with buffet dinner, USD12) on my 1st night but due to lack of cash, I'd to cancel it. A random stroll into Park Hyatt brought a pleasant surprise: every Tue, Thur, Sat and Sun there will be a free Apsara performance in the beautiful courtyard of the hotel. One can opt for the 5-course Khmer dinner (USD390++) or order wine/cocktail at 50% discount between 6-7pm. Reservation must be made as seating in the courtyard is limited, but I ordered a drink, sat in the beautiful lounge area and exited whenever the dance was on. We also chanced upon a 50% discount off delicious-looking baked goodies at the Glasshouse between 6 - 7pm.
Edit: Book seats for the Calypso Cabaret online (min 2 days before) to get tickets at 900 THB (O.P. 1200 THB).
Okay, you do not have to be fluent in every language there is but it is good to learn some basic greetings and terms, especially "expensive" and "cheap". I took to saying "thank you" in Khmer and this is very well-received by the locals. I take it that most tourists just speak English so it is refreshing for them to bargain with someone asking for a cheaper price in their local language. Always better add a smile and they will smile back :)
When traveling in a group, always be considerate of fellow travelers' interests/preferences. It is good to check with everyone if they're in agreement with your plans. Give good details when supporting your plans; I mistakenly assumed that there will be loads of street hawkers near Silom shopping area - we didn't find the market scene I had in mind despite walking a LOT. Then again, you can't please everyone so be open to criticism and other ideas! :) Also punctuality is crucial to meet important schedule points and we must respect other people's time. We missed a few shuttles buses because 1) late and 2) no one checked the correct time.
These are my top lessons learnt, I'll probably add more as I go along. Do you have any lessons to share? :)