My take on 'Ki o tsuke' is similar to the above responses.
Ki being spirit / care / awareness / alertness. In kanji - the ki that is used in 'genki' ( in top spirits / fine).
O is an object particle, often written 'wo'. It merely marks the relationship of object to verb here and has no honorific associations (that is the prefix 'O' that is found in words such as 'ohashi' - chopsticks, or 'Obasan' - Grandma. It is written differently in Hiragana)
...and Tsuke is a form of the verb 'tsukeru' which means something like to use or apply. In this abbreviated form it indicates quite a strong command and can be seen as a bit masculine I think (well - I wouldnt use it to my superiors anyway). The polite form of the phrase would be 'ki o tsukete kudasai' or 'ki o tsukete' - this is often used in Japanese in the same way that 'Take care' or 'Be careful' is used in English.
The phrase as a whole therefore means something like 'use your alertness' or 'draw on your spirit' but of course these seem a little odd in English, maybe something like 'Look alive!' is more appropriate?
I encounter the phrase mostly at school here in Japan - classes begin with a student calling the class to stand and then 'ki o tsuke!' before the bow and greeting between students and teachers. Actually - at school it is always used when asking a body of students to bow. I think it has some specific relationship to posture aswell - a little like 'attention!' in the military.
I cant recall hearing it used in our dojo at all, but then again I have not been there long.
Hope this helps