I love reading others conquests and queries, which usually come with some sound replies so I hope someone can help me reach one of my goals.  I've been working my arse both deep and wide ever since I can remember but I've always had to throw in the gloves when it comes to fitting the full 18" of my silicone double dildo inside.
I know that if you want to push yourself, it's best to have a good understanding of where your toy needs to go. The first barrier past my anus is the sigmoid. This is another sphincter muscle like your anus which is now well trained to accept any toy I've tried so far. The next resistance I get is my upward (sigmoid) colon straightening out from it's usual relaxed position. I get in there with a little deep relaxation that she calls 'The Arts'. If you just relax though, you won't get anywhere; it's more like a deep gentle pull from up inside. Once I have relaxed and adjusted to being realigned to suit my dildo's shape, I can then ease it up to the top of the lower colon but this is where things get tricky as it turns sharply, straight across from the left to right. I've found that by backing off and laying on my front left side, then slowly twisting in the last third back and forth and using 'The Arts' I can push another 3" but those last few inches always beat me. I always use a deep douche to ensure there's nothing else stopping my fun so I can only imagine it's an anatomy thing.

Any tips from experienced players who can help get me there would be gratefully received!

I'm 5'7 and I can get all but the last 4 inches of my 18 inch dildo in my ass..... The thing is, it hits a sharp turn from down to UP (meaning it's hitting the turn where the sigmoid colon meets the bottom of the descending colon).

Now if I'm at the same stage you are then I highly doubt you're hitting the turn to the transverse. In fact, most people mistake this corner all the time. If it's hitting a corner above your navel and just below your left bottom rib then yes, you are at the entrance to the transverse colon at which point you should have way more than 18 inches of that dildo in you.

Have a look an anatomy pic, it will sort it all out for you. 

I use 2 toys, 1 is 20" long double and the other is 28" i made by bonding 2 16" doubles, I also use a tool to insert them both an extra 13". I love the feeling as it goes around the bend and up the left side. and some days it goes all the way in and some days it doesn't, I use deep rinse to start - 48" colon tube and about 6 quarts of water, then when all is clean, i use about 6-8 oz mineral oil or silicone lube inside. putting lube just on the toy doesn't make it very far inside so the extra lube inside makes a big difference

100% pharmacy grade made for oral consumption... same type "Fleet" use in  their products, but i get it in 64oz containers

I take an 18" dildo in all the way often, I also take a small guys hands beyond their elbows and few have gotten to their Biceps and almost their shoulders. It is just like starting out every time you go deeper...slow and gentle play with limited pushing is how it is done. The colon will expand and it does all the time as you pass fecal matter through.  But getting it to accept things from the bottom up, takes a lot more training and mental control.  There are no shortcuts to deep play. It has taken years of slow and skilled play to take arms and Dildos as deep as i do!  I have taken a 36" colon snake (1.25")  all the way in several times...but as stated above.....some days I can and other days I just can Not!!   But it does feel so intense when it slides all the way across the Transverse colon ....! I wished I could do it all the time...but I can't!  So it comes down to Practice baby practice!   Jay  aka "Waterwagon"