Mix, bring to volume, and measure the pH. In the long run this is faster, more reproducible, and the pH check serves as quality confirmation. Some buffers, such as tris, have a very temperature-sensitive pKa. Others, such as phosphate, are very ionic strength-dependent. Effective washing will focus on disrupting these low-affinity bonds. High salt, low pH and mild chaotropic agents can be helpful.
Detergent is also helpful in prevention.
While vigorous, repeated washing can sometimes seem to solve some problems, it usually causes variation through denaturation. Albumins, often used as blocking agents, can sometimes cause the separation of already bound proteins from medium binding surfaces, increasing variability. Broad-ranged optimization strategies can yield surprising and positive results. Unless the materials are bound with an effective orientation, the simple presence adds no value. Optimum binding results from a combination of plastic surface and binding environment and is often very specific, especially for homogeneous entities such as monoclonal antibodies or recombinant proteins.