best strategy for aftermarket trades

I have been putting orders in at around 4.30pm Sydney time for the next days trading and today is a prime example of whats been going on. I put 4 trades orders in, all of them up so between them, they made a profit of 5%.

But my order was so far down the line that I bought at the peak and was sitting down 2% on open even though my stocks are up 5%. If i put my order in at 4.30pm, how could so many people got in front of my order over the entire weekend?

Id love to know how to buy after market the most effective way possible. I must be missing somthing.