Agree with most of the advice here.
As a rule of thumb I use a dildo every day in the shower or a small plug for a while if I’m working from home or whatever. It feels good, and also keeps my hole used to stretching easily - I kind of think about it the same way as I do working out at the gym. I do crunches, lift etc every other day for lots of reasons... one of which is “avoid incapacitating soreness” LOL. You’re literally just working out another muscle.
As for sex prep - the keys for me are relaxation and lube. If I have time before, like a quick shower or whatever, I give myself a few pumps with the dildo or at least a finger or 2 etc - same stuff guys above have suggested. It also REALLY helps if the top is into rimming, fingering etc. But not all of them are.
Regardless, generous lube on the cock and my hole, plus concentrating on relaxing when he goes in makes a huge difference.
Keep in mind it takes some practice to learn how to relax - you’d think it should be easy, but it’s not (until you figure out how to do it, after which it’s super easy and trust/know that you WILL figure it out and start loving the fuck out of it). Even if you think you’re “ready” your body - specifically your hole - is likely going to tense up slightly in anticipation. That’s what you have to zero in on, not letting that happen. It’s akin to getting a shot at the doctor, or better yet when you’re getting an eye exam and they do that puff of air thing. You know this isn’t going to hurt (really), and you know it will be quick, but notice how your body tenses up anyway in anticipation? That’s the same kind of reflex at work.
So understanding what your body is doing and why goes a long way to getting control of it and makes the process not as stressful. Because you realize “oh wait, there’s nothing wrong with me, this is just reflexes that I can learn to control.” That confidence will go a long way.