When I started learning to play the whistle I chose tunes based on what other people I knew were playing and which tunes I liked at the moment. But lately I've been using a different strategy, with great results.
Ever hear of an earworm? It's just a word for songs which get "stuck in your head." Not surprisingly, this started happening with tunes as I started listening to more and more Irish and Scottish music. One day, unable to get "Harvest Home" out of my head, I decided to put my affliction to good use and learned the tune. I found I was able to memorize it in about half an hour or so, instead of the large number of days it had taken me to memorize tunes the past.
Since then, I've seen "earworms" as opportunities. Whether or not a tune is on the list of tunes I think I want to learn soon, when it gets stuck in my head I work on learning the tune. I find that having the tune cycling in my head means that I'm less likely to have my own mistakes confuse me as to how the tune actually goes. This is part 1 of the basic two-step process for learning a tune.