For easy value, always smooth and engrave everything important. For almost as easy value, look for appropriately sized rooms where the walls are of incredibly high value, like iron ore or gemstones, then also smooth and engrave. Also unbroken wall seems to work better, so a room with no doors is more valuable (use stairways to get between a stack of rooms). You might want to make exceptions for really high quality or artifact doors. But they had best be
really high value to make up for the huge hit to the entire room's value for
reasons explained in more detail to here but suffice it to say it's !!!SCIENCE!!!
The final thing is literally just loading the room with expensive stuff, like if you have an artifact version of required furniture build it into the room. Or even the crude but effective method of building display cases in the room and loading them with hoity toity stuff. Been wanting an excuse to use that aluminum artifact grate? It doesn't even need to go anywhere.
When you designate a room especially of a dwarf you might see become a noble, you might want to make it bigger than you would for a peon, or if they already are a noble, bigger still, because if they get a promotion they'll want more than you can fit in a tiny room.
Some rooms are fairly time-consuming to put together, like meeting the collection of demands a monarch has, which is why you should probably have the rooms engraved already, but maybe lay off on the furniture until you know what they like. And again, spare no effort finding high value wall materials. Stuff like hematite is top notch, as is aluminum, but even less valuable ores are still more valuable than junk stone. If you're in the ridiculous scenario of having a bunch of adamantine and some of it is obviously unsafe to mine, it makes a fantastic wall material.
In any event, when you need a particular level, just keep trying these things in combination until you hit it.
Another thing I don't usually do but if you really need to and can't make the room bigger is to deconstruct the walls and replace them with constructed walls of high value material and engrave them. Be careful about destroying walls with masterwork engravings, though, it upsets the creator. This is better done before engraving as a result. Constructed walls and floors with engravings on them can also reclaim otherwise doomed-to-be-meager sand floor rooms.