Getting the tones right

Today in class, I was rather embarrassed. True that I did not get formal training for the most basic stuff, nor do I think it is necessary, but when it comes to tones, I can hear them from time to time, but really am unable to reproduce them.
As we were practicing 繞口令 or tongue twisters, the teacher made me start over and over on lines like 四十四 and so on。 later 44zhishishizi of the like. ( i will find the characters some time - hey my taking notes this semester is rather off...) In a nutshell, all i want to say there are few times that the tones are really crucial, but the number of combinations, with more less proper pronunciation makes one understandable. Context is important. if you have a map and you ask for a place named 衡芬（not that there is any to my knowledge) 在哪裡？ People will not think that you are looking for a pink object 粉紅。。。