First- LUBE LIBERALLY.
Second- TRAIN for it. The point of butt plugs is NOT to 'stretch you out' because the neck of a butt plug is particularly narrow and the interior of the colon is actually quite roomy.
The purpose of Butt Plugs is that they have a WIDE section that you must get past to get to the neck, and in repeatedly inserting and removing the butt plug, over and over and over again, what you are doing is teaching yourself how to relax that sphincter on command. It is the experience of the pain that makes your body tense up when being penetrated, which is what causes the pain. A vicious circle.
Third- the OTHER thing you want to train is your pelvic floor muscles. Once you have the butt plug in and are wearing it for a few hours at a time you will find that certain things you do will want to push the butt plug out. you will have to do Kegel flexes to pull it back in. The purpose of wearing the butt plug is to remind you to kegel frequently, and it gives you direct feedback as to whether you are holding that butt plug in or not.
Kegel exercises make your butthole STRONGER.... but they also make the sphincter more resilient. You are essentially training it to flex, and then relax.
Trust me... get a butt plug with a 2" diameter and try inserting it. It will hurt- but only as the widest point tries to get thru.
once you get past the wide part, the pain will abate and you will get a pleasurable reward of the feeling of your ass filled.
After an hour or two, pull it out, you will find it is easier to get it out that it was to get it in.
Insert it again immediately,.. try and find that same mental space and bodily angle that you had when you pulled it out, and it will slide in easier the second time. wear it for an hour or two and then repeat.
Do this every single day for a few days before your guy arrives. Over time, you will train your body not to tense up and that will result in less pain on insertion and that less intense feedback will enable you to become more relaxed still.
Also- in my experience what works best is for the Top to start by Pushing all the way in in one smooth steady stroke- not too fast but not slow- either- and then HOLD IT- just hold still with his cock fully up your ass for about 15 or 20 seconds before he starts thrusting. That gives your body time to adjust to the penetration and your asshole to relax further...
It works great for me- so might be worth a try.
I recommend the NJoy 2.0 a two pound piece of stainless steel that is perfect for anal training and has a long enough neck that it will stay where you put it. Like weight lifting for your asshole.
Keep in mind that if the pain is associated with DEPTH... that is a different issue of his dick hitting the blind pocket in the colon rather than negotiating the bend. To train for that you will need a dildo that is as long or longer than your guy's dick so you can learn how to orient yourself so his dick doesn't shove against the pocket.