Your physician should determine the cause for decreasing number of platelets and treat it appropriately.
The cause can be infection or different disorders affecting the bone marrow where the platelets are produced.
Since the bone marrow production of platelets is triggered with signal from a substance produced in liver, liver disorders can lead to low platelet count.
Your immune system may produce autoantibodies that attack the platelets leading to decreased platelets number.
Enlarged spleen due to liver disorders, or certain systemic infections, traps too many platelets and cause a decrease in your circulating platelets.
Use of certain medication can bring on this problem (heparin, anti-inflammatory drugs), too.
Only treating the underlying cause, will lead to increasing the platelets number back to normal.

Best wishes!
Marija