When I got ET it was mostly based on scores from Asvab, nuc test, grades (but that may not be the case anymore). One thing I can say for sure is if you want ET make sure you list your preferences as ET/EM/MM. Or you will end up EM or MM as they are always happy to give your second choice of MM since they need tham at about 6 to 1 ET ratio, like wise if you put EM first they will make you "happy"since they need 3 to 1 ET ratio of them... If you get the picture put down ET/EM/MM!!! (not that any one of them is bad, it just depends on what you like to do) And do not despair if you get one you do not like be the best at that and you will still excell in life!!
Rob