I would use the phosphate buffer used for elution. After solving the protein in this buffer youcan either centrifuge or filtrate the sample to remove any insoluble compound, which could block the column. If you dissolve your protein sample in any other buffer, it might happen that once the protein is in contact with elution buffer it precipitates becasue the solubility is different.
Also remember that BSA is an equilibrium between monomer and dimer. You might want to choose a protein that performs nicer (only one quaternary state).