A couple of more tips from someone who has completed PLC Juniors and Seniors....
First off, practice inspection arms ALL THE TIME ON YOUR OWN from the moment it gets taught to you. Ask the priors in your platoon to watch you and coach you. The two most important things in an inspection are, in order, 1) your rifle and 2) your bearing. Hygiene/uniform/knowledge are DISTANT runners-up. If you have extra time before an inspection and can't decide whether to spend it on your rifle or your uniform, spend it on the rifle. The Colonel might turn a blind eye to an I.P. here or there, and he won't be taking off your cover to check for sweat stains. He will, however, go absolutely RIPSH!T if he sees even a speck of rust on that M16A2 weapon system.
As far as bearing goes, everything that you say should be said in a tone of absolute certitude. If you get one of those impossible knowledge questions--and I guarantee that you will get at least one at OCS, at some point-- don't hem and haw, respond immediately, "Sir, this candidate does not know but will find out." Another bearing hint, don't fall into the inspection arms trap. That's when the inspector will ask you a question either while you're at port arms waiting for him to grab the weapon out of your hands, or while he's about to hand the weapon back to you. In an inspection you ONLY TALK AT ORDER ARMS. During my Company CO's inspection this summer he asked one candidate, "Why do you want to be a Marine Officer?" as he was performing inspection arms. Poor guy stood there at port arms for about 30 seconds before the Major grabbed the rifle out of his hands. He did the right thing, which is NEVER TALK AT PORT ARMS even if you have an impatient looking Company CO staring at you awaiting an answer.
Also, for all you returning PLC Seniors, here's a little trick. You are expected to return to OCS with all of your issue utilities and boots. However, they will be making PX calls for missing gear, which, if you lost any from the previous summer, you will have to purchase at your own expense. USE THIS OPPORTUNITY to buy an extra inspection cover, and also a blouse and trousers if you want to drop the extra change. I highly recommend having an inspection cover simply because your everyday cover will be nasty no matter how many times you wash it. It's pretty convenient to have a pristine cover, perfectly marked and shaped, sitting in your wall locker that you can grab right before you head out to the parade deck.
Finally, like other candidates have said, don't sweat it too much. Seriously. In the grand scheme of things at OCS, inspections are pretty insignificant.