Ever written the gojūon?
http://en.wikipedia.org/wiki/Gojuon
If you haven't, you should learn it, because it is used in dictionaries. If you already know the order of the characters, it's just a matter of writing it out that way.
If you do, you will notice that シ is written to the upper right of ツ. Notice how the final line is drawn away from that central point between the characters. (I hope you are paying attention to stroke order and direction). Also notice how the two shorter lines usually seem to point more at the end of that final line. Now that you know the final lines are drawn away from that central point, and you also know that the s-row is to the right of the t-row, it shouldn't be that hard to remember which is which.
Now where's my hug?