Learning to bottom is like training for a marathon, you can't be a weekend warrior. You need to keep at it until you can take whatever it is you want to stick up there.
First, here is what not to do. Don't try poppers until you know what you can take. Don't use devices that mechanically stretch your ass: like speculums. Both of these will get your ass to open, but both can also cause damage without you being aware of it.
The key to getting yourself to open up is to try different things. Lube up your finger and play with your sphincter muscle. Don't just jam it in there, massage it, tap it, love it. Find what gives you the best sensation.
Toys are very useful. Here is an anal training kit
. It includes three butt plugs in progressively larger sizes, the first being slightly fatter than a finger. A plug will get your muscles used to relaxing and you can work up to the larger sizes. The one essential toy to this is lube. Some guys can sit on a greased up fire-hydrant no problem, but the same guys would have trouble getting a few fingers up there without momma lube.
Let us know how it goes.