When leaving a table, before presenting your chips for coloring up, ALWAYS pocket a few chips of the denomination just BELOW the largest you are playing. Win or lose. Don't do this surreptitiously, but try to do it before the PC's attention is drawn to the table. Ploppies do this, so it doesn't look unnatural. You're just saving some chips that you plan to play later, right? In my experience, this works just fine. Maybe the PCs see it, maybe they don't. If they don't, they'll probably calculate your win or loss incorrectly. There's little down side. Do the same thing when changing tables, and if you run out of chips on the felt, don't pull out the chips in your pocket, buy in again. I don't think that it's the Eye's job to look for this kind of thing and report back to the PCs under normal circumstances.
Request year-end win-loss statements, and see if this hasn't worked out for you.
This should go a long way toward concealing the sizes of your cashouts. If you're making a high-value play, it may be inadequate to cover your expectation. You may want to add in surreptitious ratholing during play. As others have said, this CAN backfire. From published stories, surveillance people seem to be very familiar with the down-the-back-of-the-shirt move. I would suggest, instead, practicing palming chips, and in a very natural, inconspicuous way pocketing them. If you're caught doing it this way, at least you have plausible deniability. Ploppies rathole, too, but not down the back of the shirt!
If you're doing a lot of high-value play, you may have to engage in deliberate "loss-generating" plays to rehabilitate your player's card. Or it may become impractical to do so, meaning you can't afford to get too well known as a winner.
If you have any sizable stash of chips, don't cash them at the cage all at the same time. The news could make its way back to the pit.