Rule 5 of Work: Underpromise and Overdeliver
If you know you can do it by Wednesday always say Friday. If you know it will take your department a week, say two. If you know it will cost an extra two people to get the new machine installed and up and running, then say three.
This isn't dishonest, merely prudent. If it gets spotted that this is what you do, then openly and honestly admit it and say you always build in a contingency percentage into your calculations. They can't kill you for that.
That's the first part. Underpromise. And just because you have said Friday or two weeks or whatever doesn't mean you can coast and use up that allowance. Oh, no. What you have to do is make sure you deliver early, on budget, and better than promised. And that's the second part. Overdeliver. This means if you promised to have the report finished by Monday, first thing, it is finished but not only is it a report but it also contains the full implementation plans for the new premises. Or if you said you'd have the exhibition stand up and running by Sunday night with only two extra members of staff you have—and you've managed to get your major competitor to pull out of the show. Or if you said you'd have a rough proposal written for the new company brochure by the next meeting you not only have this but also a full color mock-up, the complete text written and proofread, all the photos taken and full printing costs and quotes for distribution. Obviously you've got to be careful that you don't overstep the mark and assume responsibilities you haven't been given, but I'm sure you get the idea.
Again it might be stating the obvious but don't be too blatant when you do this or your boss will get to expect it—it should be a pleasant surprise not a frequently used tactic.
It also helps sometimes to act dumb. You can pretend you don't really understand some new technique or software when in reality you know it back to front. Then when you suddenly do all the budgets on the spreadsheets that no one else could, you look good. If, in advance, you had said "Oh, yes, I know that, I worked with these spreadsheets at my last place," there is no surprise and you've given the game away—and your advantage.
When you underpromise and overdeliver you have to have a bottom line—in your case, as a Rules Player, it is simply that you will never deliver late or deliver short. That's it. If you have to sweat blood and work all night then so be it. You will deliver when you said you would—or earlier if you can—without exception. It is better to negotiate a longer delivery time in the first place than to have to let someone down. A lot of people are so keen to be liked, or approved of, or praised that they will agree to the first delivery time offered to them—"Oh yes, I can do that," and then they fail. They look like pushovers in the first place and incompetent in the end.