If you are trying to express non-bacterial protein, it is quite likely that your gene of interest contains rare codons and thus E.Coli can't express it well. Rosetta and Rosetta 2 cells (available from Novagen) are designed to overcome this problem. For human proteins, expression levels may sometimes change from "I need to enrich the protein batch on affinity column before I can see protein on the gel" to "80% of the protein in the lysed cells is my protein of interest".
Baculovirus-mediated expression in Spodoptera frugiperd and Trichoplusia ni cells
Chinese hamster ovary (CHO) cultured cells
Human Embryonic Kidney (HEK) 293 cultured cells