If a partner isn't available, don't be afraid to use a toy.  If you don't feel like doing the work manually, and no partners is available, you could always look into something like this:
http://www.kenstwistedmind.com/Fucking-Machines/PRM.html

Grease.

Lots and lots of grease.

Practice, Practice, Practice

Re: taking the BIG one

The best way is to gradually get used to big dicks by using progressively larger and larger dildos until your ass adjusts to the larger size. This can take several weeks. The trick is to go slowly; if it hurts, stop. The key is to stretch the sphincter but not damage the muscle.

There is a book on anal sex which I found to be very usefull called Anal Pleasure and Health. It explains everything you ever wanted to know about getting fucked in the butt. You can probably order it on the internet. I bought a copy at Barnes and Noble.  

I've been using dildos on myself for several years. When I first started, even a small dildo was painfull. But after a while, the sphincter relaxed and I was able to use larger and larger ones. I own a realistic dildo which is an exact replica of John Holmes' cock and I can use it without pain. Once you reach this level you can accomodate any TS provider and not have to worry about it.

When you have a session as a bottom it is still important to go slowly until the sphincter loosens up; for me this takes about three to four minutes of gradual pressure from the penis.

Of course it's important to use lots of water based lubricant.

Have fun and and write a review!