I top 98% of the time which means I hardly ever bottom and tighten up quite a bit in between unless I play with toys solo. Initially I trained years ago with a S/M/L butt plug set and tons of lube gradually increasing the size.
So two things. One is that once I learned how to relax sphincter muscles I've never forgotten. It's easier to do all over again compared to the first time.
Second is that when I haven't bottomed in a long time I will use plugs a day or two in advance to quickly train my butt again when I know I'm going to bottom.
Okay. Third is that this includes using a dildo or butt plug as a toy immediately before being penetrated--like during foreplay--when at the last minute when I didn't expect to be topped.
As always I recommend using a lube injector.
Bottom line (pun intended). I want a top to get into me easily without injuring himself or me... and without taking forever.
Have fun!