I actually found another mnemonic. You see there are a whole bucnh of in's as in vivo, in vitro, in silico and in utero etc. The last two are fairly self-explanatory. It's the first two which were most troublesome and coincidentally most important.
Anyway the mnemonic is:
vivo means alive like vegas.
Since I always knew one (of the in's) was in a test tube and the other was in a live organism so it will be easy now to distinguish between the two.