I think riding a comfortable yet sporty handling bike helps. I used to ride a custom 531C bike that was great, but since early 2015 I’ve ridden a Specialized Roubaix, which is probably even better. Being full carbon makes it light and responsive and the geometry and carbon seat post give added comfort. A responsive bike makes being more inclined to get out of the saddle occasionally and give my backside a rest. I upgraded the saddle to a Specialized Toupe, which despite seeming like a rock hard racing saddle is amazingly comfy for long rides. It has a cut out to relieve pressure where it matters.
I find MTB shoes and SPD pedals most practical and best suited to Audax rides and err on the large side to allow for expansion room and thicker socks.
After hurting my hip, I flipped the stem, which raised my handlebars and regained my comfort. This means that my hands put less pressure on the bars, and I have gel strips under thick cork type bar tape and wear gel mitts for further protection. I usually have clip on aero bars for extra changes of hand position but managed without these on 2015 PBP and 2017 LEL. I missed them heading south on the fens of course.
I don’t wear bib shorts and mostly wear dhb Aeron shorts. I don’t think I’ve ever paid more than £30 for a pair of cycling shorts. On the 2017 LEL, I wore the same pair of old CoolMax shorts for the whole ride, despite carrying spare shorts and having more pairs in my drop bags at Thirsk and Edinburgh. This was because I was comfortable and didn’t want to risk changing them for a different brand. I usually ride PBP in one pair of shorts too, but carry a spare pair in my bag just in case.
I took some crocs to wear at controls on LEL. Like others, I had a wee a few km before controls if I needed one, if there was somewhere discrete and convenient. Otherwise I waited until the control.