Been a few times, and to Malaysia, Vietnam etc etc - living in Japan and having an office in Hong Kong meant lots of free company travel!
Anyway advice?
1. Don't be frightened to say no, be polite, respectful and smile and decline and walk away. Vendors, sellers, shop keepers, strip joints will all try to tempt tourists in - be polite say no, smile and move on.
2. Use the ATMs for money, much easier, draw out small amounts and store the money on you in a couple of different places so if someone does lift your wallet you have cash tucked in your sock or the like!
3. Make good use of the tourist information centres, even if you have read about something in Lonely Planet (remember the writers of those books always get great treatment and prices because they tell the tourist places who they are!) check with the tourist offices who are generally not on the take and will be very impartial.
4. Try eat with the locals don't always default to McDonalds, actually if you are not afraid to try their food, understand their ways, you will get treated much better.
To be perfectly frank, the biggest bit of advice I could give is that they are wasting 5 weeks sitting in Phuket. It is the Thai version of Benidorm. They are flying all that way to sit in one resort for weeks on end whilst the likes of Krabi, Hat Railey, Koh Pang Nang, Koh Samui etc etc are all within easy reach and much better places.