A lot of the suggestions given already are great. I am 100% bottom and incredibly tight.

• Use a good lube (and no - Astroglide and KY are not good lubes. They have both been shown to increase damage to anal tissues, thus increasing the risk for infections). Silicone-based lubes are condom safe and stay slick for a long time,
• Always prepare yourself before an anal adventure. I usually clean out (read enema), use a buttplug and/or dildo to comfortably start to open up, then clean up with a hot bath.
• Your top should be patient. If you are having problems, ask him to lie on his back so you can control the entry. After you have taken care of that part and ride him for a little while, he's free to flip you over and take control for the rest of the time.
• Relax. Try not to think about it being difficult or painful.
• Get him to rim you for a while to get you hot and ready. Trust me, when you get hot and bothered, his cock is going to go in much easier.
• Using a buttplug or dildo prior to an anal session is not going to affect your tightness, it just helps you get some of the initial discomfort over so you are ready.

By the way - congrats on learning the milking (or as I call it, the butterfly) technique. If you have that level of control with your ass muscles, you will always be able to make things as tight as you want.