You need to use a thinner dildo - and make sure it is very soft and flexible.
I have a 23inch double-dildo from Bondara in uk ( http://www.bondara.co.uk/dildos/double-dildos/1257/natural-double-dildo.html )
I can take it almost fully :)
First you need to be cleaned out completely..
Then u need ALOT of lube - and i mean ALOT - squirt some up your butt.
Now - when you hit the dead-end, pull back approx a small inch - bear down as if you were goin to the bathroom, and then push - slowly but firm - you will feel that the dildo will make a slight turn - and swoop, you're in your sigmoid :)
This technique is self-taught and afterwards confirmed on other forums :)