The Holy Grail Tip that solved the problem for us: If it hurts or feels uncomfortable, hold it there for a minute, then take it OUT for at least a minute before trying again. This one tip was key for us achieving success. For some reason, the muscles relax a lot more and faster after you remove it. That tip was essential for us! You can kiss, or massage, or touch, or masturbate each other during the minute waiting time, or dilate while watching porn, or even dilate earlier the same day alone. There are lots of ways to make it fun.
The length and girth problem: Using thinner (but not shorter) soft jelly dildos to dilate before trying with his cock - while using the same "hold still for a minute, then remove for a minute" technique solved it. I think the very first time we tried this, we used 5 dildos of increasing size. The very next time we used 4. Now we're down to 2-3 before we can try with his cock, and it goes faster.
Things that didn't work for us: "Don't take it out," was the worst advice ever, and the exact opposite of what did work. "Keep it moving," also failed to ever make it pleasurable, and made it continue to hurt more. If we had tried, "just lots of lube and time," we'd still be waiting to have anal sex. I've never found his fingers to be comfortable back there. His hands are too rough and his knuckles are too hard! I really don't like his fingers in my ass. Also, butt plugs didn't work at all, no matter how many different kinds and materials I tried, but everyone kept recommending them! They're narrow where they need to be wide and too wide where they don't need to be and never long enough unless the girth become ridiculous. Dildos or something at least vaguely penis shaped worked best. Also, hard materials like a lot of those extremely expensive silicone dildos made my ass too sore for anal sex, which defeated the purpose. Sure, those cheap clear jelly dildos are a lot softer than your cock, but at least his/her ass won't be too sore to enjoy your cock after you've loosened that ass up!