What is the test for determining a sentence that is not constitutionally disproportionate?

"In view of the substantial deference that must be accorded legislatures and sentencing courts, a reviewing court rarely will be required to engage in extended analysis to determine that a sentence is not constitutionally disproportionate." Solem v. Helm, 463 U.S. 277, 290 n. 16, 103 S.Ct. 3001, 3009 n. 16, 77 L.Ed.2d 637 (1983). Nevertheless, "as a matter of principle ... a criminal sentence must be proportionate to the crime for which the defendant has been convicted." Id. at 290, 103 S.Ct. at 3009. In deciding the constitutionality of a sentence,