Last time I checked the standard Android encryption will not do the sdcard partition (I mean not the physical card, but the partition on the internal flash, usually the biggest chunk of it, like let's say 11 out of 16GB). YES, some manufacturers like Samsung and Motorola (possibly many more) have their own solution (I bet a really crappy one but never mind that) and it would do mostly everything, including the big sdcard partition and (if needed) even the physical sdcard.
Anyway bottom line is that:
a. depending on the phone you might not be able to encrypt at all
b. ANY activity, including storing random (non-private) crap on the phone and then removing it helps. However, this is no maggic bullet.