There are various indicators that could point towards picking a...

There are various indicators that could point towards picking a bottom on a downtrend. I will take Blackmores for example here. If you notice, the SP has been sitting in the lower band for quite some time and every time it tries to breakthrough the $116-$117 mark it forms a cross, what you call a Doji. This essentially means that the buyers are not ready to pay more than the current price.

The next indicator is the 20, 50 and 100 DMA. All three MA are downward slopping with the 20DMA acting as a strong resistance. The SP has not manage to break through this level so far. I also use Parabolic Stop and Reversal indicators to indicate my buy and sell points. Currently this indicator is pointing towards sell.

In regards to finding the bottom, no one can pick the absolute bottom but the common resistance points are 1) Any gaps left while the SP was in a uptrend act as a strong resistance, on its way down 2) Market has a tendency to be in love with round figures, means any multiples of 50 act as a strong resistance. This indicates that the next resistance for Blackmores will be $100 and probably bounce from there. 3) You can also see past charts and try to identify any points where the SP kept bouncing off which may be a good point to take profits when you are shorting.

Hope this helps. Good luck.