A good URL is one that has special meaning for you, at least that's what I think. It doesn't have to be mushy-gushy sentimental meaning, but just one that means something. It can be funny, it can be sentimental.... whatever works for you. I chose "mjyouaremylife" because "You Are My Life" is one of my all-time favorite songs. Not just by him, but in general. The lyrics, the musical composition, it's all just beautiful and as many times as I've heard it, it still makes me cry. I can even apply the lyrics to my own life and for that alone it carries special meaning. So I went with it.
Or there is also the option of just going with something totally 'off the wall' (pardon the pun) and random.
But I think the best URL's are those that reflect the main theme/content of your site. I mean, for example, I wouldn't use 'mjyouaremylife.tumblr.com' to be the address for a website that's all about my travels through Europe, ya know? It's misleading, so it is best to at least keep your address related to the content of your site.
Speaking of... I haven't put anything in my tumblr for quite sometime. I'm thinking it's about time to start adding to it again.