I look after a showroom no bigger than 6 20ft containers and have to display well over a thousand products. It constantly looks cluttered no matter what i do because the owner wants absoulutly everything on display. How can i make it look less cluttered?

Use negative space - tiered shelving to use as much of the wall space as possible.

use vertical space as much as possible

What type of showrooom is it? where i work its for cabinets and we door styles and tile ect. on the walls as well as having floor modles

With a can of gasoline and a match

suspend things from the ceiling.

Clump.
Make sure you can leave some "white spaces" between displays of similar items, by grouping similar items together.

In other words, have a set number of categories for items - less than a dozen. Group all the items into these categories. Then display them together. Your boss should have no problem, as you are following the orders to display everything - you are just making it easier for the customers to compare similar related items (sizes, colors) and then move on to the next group of similar items.

Don't forget to go up and down vs. right and left with your displays whenever possible.

Also, if you are able, post a visual map near the entrance that alerts people where each section of product is. Have a matching sign at the product section ("Hardware," etc.).

Best of luck!