Tip 1: When in doubt, put in a giant chandelier. A centralized and well placed light fixture can liven up even the drabest of throne rooms. And if it happens to look like a rocket ship made of diamonds, then so much the better!
Tip 2: Get creative with table placements! Having a host of foreign dignitaries over for brunch? Try a U-shaped dining arrangement to maximize surface area and accommodate those larger international delegations.
Tip 3: Just add a piano. Too many empty drawing rooms? Turn one into a "music" room, by just adding a large and perfunctory instrument, such as a piano or a harp. For maximial pretentiousness, choose a cherished museum piece from your private collection and then don't allow anyone to play it.
Tip 4: Play around with color. As in, a single color. Try making everything in the room almost exactly the same shade of a single gaudy hue such as gold, silver, or purple. Nothing says "regal" like monochromaticity.
Tip 5: Keep the entryway simple. You've already got 20 acres of manicured gardens maintained around the clock by an army of underlings. No need to go overboard with the front door. If needed, spruce it up with a pair of colorfully dressed guards.