There are kits available for enrichment of phosphoproteins. I don't have personal experience with them. Maybe someone can comment on how well they work.

Here are a few of the products:

https://www.thermofisher.com/order/catalog/product/90003

http://www.clontech.com/US/Products/Protein_Expression_and_Purification/Purify_Phospho-_and_Glycoproteins/Phosphoprotein_Enrichment_Kit

Here is a paper describing the use of metal ion affinity for phosphoprotein enrichment:

http://www.ncbi.nlm.nih.gov/pubmed/17480206

Thank you very much Adam,

I am already using TiO2 magnetic beads (Thermo) to enrich phosphopeptides from serum. I wanted to know what is the best way to enrich lower abundance serum proteins (and deplete albumin at least) before phosphoprotein/phosphopeptide enrichment in order not to interfere with the phospo- enrichment process and have a high yield of phosphoprotein identification by LC-MS/MS

There are materials available commercially for depletion of albumin from serum-containing medium. Here are a few examples:

http://www.abcam.com/bsa-removal-kit-ab173231.html

https://www.thermofisher.com/order/catalog/product/85160