All good advice, here's some other tidbits.
Don't volunteer for anything, esp. if you don't know what you're volunteering for.
If you ask for care packages ie. sweets and baked goods, be sure that whoever is sending them sends enough for your whole platoon and not just you. Your platton mates will appreciate it and you DI(s) might bend you if don't have enough for all.
Scribe duty is good if you can get it, at the least it will get you of field day duties. If you're good with the head scribe or are the head scribe you can pick your firewatch shifts.
If you ever get your choice of firewatch shift I'd choose either the first or the last shifts. The first, you're already dressed and awake so there's no waking up early and having to change back into your cammies. Last shift, you're already dressed, your rack made, teeth brushed when reveille sounds so you avoid the morning rush to get ready.
Go to church every Sunday if you can, even if you're not religious, it gives you a bit of a break and a change in scenery. You can't totally relax but it's probably the most relaxed you'll ever be in your entire time at MCRD except for maybe the night before graduation.
When/if things get down just tell yourself it's only 3 months, it's only months, or however much longer you have to go.
Don't fall behind in anything if you can at all avoid it. Motivate yourself by telling yourself that you're not going to spend 1 day longer than you absolutely have to at MCRD. That's what helped keep me going and kept me from getting droppe, I was not going to get sent to PCP or MCP no matter what.
Study your prac whenever you DIs give you the time to do it. It may not be that interesting if you're not a reader or a history buff but you can get dropped for failing your prac and considering that it's not that hard there's no reason for you to get dropped for something so simple.
One last thing, be prepared to be up all night and all day on your first night at MCRD. You'll get dropped off at night and you'll be up all night and all of the next day playing the hurry up and wait game. You don't get to sleep until the next night so you'll be up for nearly 24 hours straight so be sure not too stay up late at MEPS the night before.
"Gone to Florida to fight the Indians. Will be back when the war is over."
Colonel Commandant Archibald Henderson, USMC
in a note pinned to his office door, 1836
"We have two companies of Marines running rampant all over the northern half of this island, and three Army regiments pinned down in the southwestern corner, doing nothing. What the hell is going on?"
Gen. John W. Vessey Jr., USA, Chairman of the the Joint Chiefs of Staff
during the assault on Grenada, 1983