So if we start on the primise that the bicep is reall
y just a muscle that assist the back in pulling things (which it is), generally speaking, the best way to increase the size of the biceps is to start really hammering the back with pulling movements. The body has a tendancy to not let the biceps get too out of proprotion to the back.
Like you, I have 18 inch bi's but I haven't done a curl in over a year, I just do a hell of a lot of back work. Assuming your arms are 18 inches from mostly muscle (and not fat) you may be advanced enough to the point that extra bicep isolation work might help. However, the majority of people who complain about bicep developement just need to increase their back work.
I highly reccomend adding some close grip pullups to you back day (palms facing you, pinkies touching). It's a great pulling exercise that hits the bi's pretty well.
The "peak" people talk about is mostly a genetic quality that come about from having a shorter belly in their bicep which allows the muscle to bunch up when contracted. People with longer muscles aren't going to be able to get their bicep to bunch up like that.
Good luck bro.