I was first fucked by a white daddy with a huge dick. At first it was really agonizing, and that's on top of the fact that I've played with a big dildo before so it shouldn't be problem but it did. What I did after the first unsuccessful attempt was to go to the toilet, clean the area again and try again a second time and it went in pretty much completely. I guess what I was trying to imply was that attempting anal is partly a mental process where the bottom has to relax both his anus as well as his thought. If he's too tense no matter if you have a big or small dick it will never work out. If all else fails, try smaller objects first such as your fingers and slowly work your way up. Lube works wonders too, use plenty of it.
Oh! And like what the guy above said, only penetrate with an erect dick. If you're flaccid or it's a semi it will never be able to penetrate!