Bombast from afar: apply daily as needed. (Consult physician for appropriate dosage.)
The Thesaurus won't help you become a better descriptive writer, but reading good descriptive writing sure will, as will practicing to better your craft: this means reading the kinds of descriptive writing that you enjoy, and paying attention to it, so that it both informs and reinforces your own writerly proclivities. In other words, you're mostly right-on with your approach, but bear in mind that the best descriptive writing operates in one of two ways. It either shows something in a new and startling way (a description is so new and unusual you just
have to stop and gape and say, "Wow! I wish I'd thought of that!"), or it performs tasks beyond the merely descriptive. The goal of the latter isn't merely to catalogue sensory details, but rather to use those details to impart much more than mere surface impressions. Good descriptive writing resonates with many other parts of the story, whether a person, a place, a thing, a theme...
Oh but what in the world does that mean? Good descriptive writing accomplishes more than picture painting with words. Hammer, meet head of nail:
...show what [you] need to show without telling it point blank to the reader...
This is the great big magic act of writing (well, one of them, anyway), and Kate Thornton knows what she's talking about when she says that. Pretty much the same thing was said by John Gardner in his
book on writing, which was emphasized with a little exercise that I'll try to reproduce below (though likely it's a botch). Exercise goes something like this:
Describe a scene in which there is a man and a barn, and the man has just lost his only son in a war. Do this without mentioning war, death, loss, or the son.
If you manage to do what Gardner asks (or, in you own way, manage to impart a great many other things beneath the iceberg-tip of your descriptions), you will very likely produce meaningful description. Telling description. Emotionally resonant description.
If you manage to write something like this...
The door irised open.
...you will have hit the nail of descriptive brevity and wonderment straight on the head, just like Heinlein so famously did with that very line. No one had ever said that before about a door before he did it, and the description succinctly transmits what any good old-fashioned science fiction door should do: open in its own, new, and very special way. Which is a way that made many say, "Wow, I wish I'd thought of that!"
Next trick? Practice. There's no substitute for hard work.