First, when purchasing Trip Insurance make sure you fully understand the coverage offered by the policy you are planning to purchase to confirm it covers the type(s) of loss(es) you wish to insure against.
A good place to do this is a
www.insuremytrip.com where you can compare the coverages and premiums offered by every Travel Insurance carrier.
Most policies have lower premiums if you purchase the insurance within a set period of booking your trip (usually within 10 days). If you purchase the policy outside that window the premiums are higher and, in the case of medical cancellations, may not cover pre-existing conditions.