I always douche. I'm a bit paranoid about it, to be honest, but I'd rather spend 10 minuts sorting the problem out before it starts. Top tip: douche several times, using not too much water. Don't feel that you have to completely empty the bulb when you're squeezing it - they hold more water than you think which just ends up further up your colon. This can lead to a mess later on when you aren't expecting it. If your partner has a 6" long penis, then you only really need to wash a few inches more than that. Don't go for a full on colonic.
With regards to full on penetration, try it with a toy first. Maybe use somethign like one of the Joel Kaplan prostate toys. Don't rush, whatever you do, and use plenty of lube. The toys I've seen are generally smaller in profile than a penis, so you can get used to the feeling of something being inserted without it stretching your sphincter too much.
The first time he penetrates you, make sure you're the one who is in control, ie, on top. You'll be able to feel your body accomodating him rather more intensely than he will. If you're feeling pain, you're going too far or too fast, or maybe both. Don't feel that you have to achieve full on penetration the first time you try. Also, most of the pain you'll likely feel is your external sphincter expanding. Once you're past that, it gets much easier.
There are numbing sprays and lubes that you might consider trying. Personally, I'd rather feel the pain so I can call a halt to proceedings and figure out what is going wrong. As others have said, use lots of lube. Your body doesn't provide any natural lubrication in this area, so make sure you have plenty added. There are special anal lubes that are thicker than normal ones. Tracy Cox's version is good. Use about twice as much as you think you need, until you get used to things.