That's an excellent question. Last time I was in the pit at a coldplay show - it was at Nissan... I didn't arrive super early, and just hung toward the back of the pit and watched an amazing show... without some jackass sweat-humping me from behind because everyone is surging to the stage.
If, however, you need to be front and center on the rail - it becomes a little more difficult. I had pit tix to a NIN show (also at Nissan), and to get the rail on that night, you had to be in line at the gate SUPER early. You end up getting in a single file line, and walked to the stage before the show. At that point, you have hurry to claim your real estate and post up - stick it out without relinquishing your spot for the entirety of the concert. Definitely not my style, but - if you're a wear-the-band's-shirt-on-the-day-of-the-show kind of fanatic... Get in line embarrassingly early.
Just a suggestion. See you all there.
edit: Side note for the fanatics: On the day of that NIN show - Nissan let us in to watch the soundcheck (where Trent played a few songs just for us). Not sure if Coldplay is equally generous to their audience, but something to keep in mind.