Thai people really appreciate it when you try to speak their language, even a little bit.   You really don't need to know alot to communicate, and some basic phrases can get you through almost any situtation. 
Here are some phrases and words that we would recommend you memorize.  Don't worry if you don't prounouce them exactly correct, as a Thai person knows you do not speak Thai and they will give you great latitude. 
And don't be surprised if the reaction is one of friendly laughter and gratitude for giving it a go.
Some pronunciation notes for the table below:
"ai" is prounouced "eye", "oo" is prounounced "ou"
The word "krup" for males, and "kha" for females can be added to the end of most phrases which is a show of respect for the other person.
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General Phrases Male Female
Hello (greeting and parting) sa-wa-dee krup sa-wa-dee kha
How are you feeling? sa-bai dee rue krup sa-bai dee rue kha
I feel fine (answer) sa-bai dee krup sa-bai dee kha
I do not feel well (answer) sa-bai dee my krup sa-bai dee my kha
I cannot speak Thai poot thai my dai poot thai my dai
Thank you kop koon krup kop koon kha
Never mind (friendly) mai pen rai mai pen rai
I do not understand my kow chai krup my kow chai kha
How much does this cost nee towl-rai krup nee towl-rai krup
Very expensive paeng maag paeng maag
Sorry or Excuse me kor thoad krup kor thoad kha
Where is the toilet hong nahm yoo tee nai hong nahm yoo tee nai
Beautiful, pretty su-way su-way
Yes chai (krup) chai (kha)
No mai (krup) mai (kha)
Directions Male Female
Stop here (useful in taxi) yuut tee nee krup yuut tee nee kha
Slow down cha-cha cha-cha
Left turn leeo sai leeo sai
Right turn leeo khwa leeo khwa
Go straight trong pai trong pai
Drink Male Female
Water nahm nahm
Ice nam khang nam khang
Tea (hot) nahm cha long nahm cha long
Tea (with ice) nahm cha khang nahm cha khang
Coffee ga-fae ga-fae
Food Male Female
Chicken gai gai
Beef nuea nuea
Pork moo moo
Fish plah plaa
Prawn goong goong
Spicy prawn soup tom yahm goong tom yahm goong
Spicy pet pet
Not spicy mai pet mai pet
Rice (white) khao khao
Rice (fried) khao pahd khao pahd
Delicious ah-roy ah-roy
Very delicious ah-roy mahk ah-roy mahk