I am a non-European passport holder resident in the UK. I have applied for a Spanish short term visitor multiple entry visa for the last three years. They had only given me up to a one month visa with single entry, even though I informed them that I was looking to buy property in Spain last year. I never had any problem getting multiple entry visas to Italy, France and Portugal before when I only booked one holiday. Now I have bought a property in Spain with my partner. I would like to visit Spain more often with my family when the flight ticket price is good. How should I apply for a multiple entry Schengen visa in the Spanish embassy?
You apply for a multiple-entry Schengen (short-stay) visa exactly in the same way than for a single entry Schengen visa, filling in the form accordingly. Your best bet is continuing to apply for a visa for each holiday. Once you have been there several time and can show that you used your visa appropriately, you will have a better chance to get a multiple entry visa. Being a UK resident should be in your favour too. Finally, you could write a letter to highlight all this and make a case for that visa and include it in your application.
But which visa is granted is mostly up to the Spanish consulate. None of this provide any guarantee.