Ja, OK, let's say you have a 3x9 drivetrain - 3 chainrings and 9 cogs on the cassette. In general, most of your trail riding is done while in the middle chainring. On most stock setups, your chainline is such that you can use safely & routinely use all 9 gears in the back when in that middle ring, no problems. However, when you are in the Big Ring (literally the biggest chainring) up front, you should ideally try to stay in the 3 smallest cogs in back (smallest in diameter, least amount of teeth). Likewise, if you are in the granny ring up front, you should try to stay within the 3 biggest (closest to your wheel) cogs in the back.
This will ensure that you have a properly straight chainline. Stand behind your bike and look at your chainline from the cassette up to the chainrings. It shouldn't "angle" too much either towards or away from the bike. Standing there, you can picture when you are doing big-big or small-small, the chainline is at a hefty angle - hence the term "cross-chaining". Not good. Your goal when shifting is too keep that chainline as "straight" as possible. Cross-chaining is hell on your drivetrain; it puts a lot of undue stress on the chain, and it wears down the teeth on your cassette and rings alarmingly fast.
On a 3x9 drivetrain, there aren't really 27 completely unique gearings - closer to half that number, in fact. That's one reason why 2x9s and 1x9s (or single-speeds, haha!) are increasingly popular. In theory, you can use all 27 gears, but in practice, you definitely shouldn't. Your drivetrain will explode in short order.
Also, you are pretty big dude, so that adds stress to the drivetrain - that itself isn't a big deal, but it doesn't help either. Here are some tips to consider: When you shift gears, do NOT do it "under load" (in the middle of a heavy pedal mash). Either pause your pedal stroke for the split-second when you shift or just unweight your pedal stroke during the shift. Also, anticipate shifts ahead of time - trying to downshift while struggling to grind up a hill is going to end badly. Remember that your derailleurs are just brute-force mechanisms - they don't operate well under a heavy load (or hard pressure, or however you want to think of it). Higher-end systems (XTR or X.0) are indeed much more forgiving and consistent, but nothing's bomb-proof.