Use as fresh bud as possible, the older it is the darker the run will be. Also, keep temps as low as possible while blasting, higher temps will pull more chlorophyll.
szuko has good advice as well.
Not much that you can do to get a bigger yeild as far as I know. You could try running more solvent through the product to make sure you have extracted as much as possible, but really to increase the yield you need either more product or better quality product. I'm sure being careful not to waste anything will help as well but the big difference is in product. If you blast some premium nugs you will get some really good oil in all likelihood, but you can still get some really good oil from mid grade bud.
If by everclean method you mean filtering/winterizing, then expect to lose some in the filtration process. You lose weight in the dewaxing process.