I also try to keep intros short these days for the same reasons as Boydie pointed out. I actually like long intros myself, take hotel California for example, that wouldn't be the song it is without the intro.
I appreciate though that in this modern age of short attention spans and immediate gratification, they're not in vogue.
I often take my original intro, halve it and there it is.
I also now often end up adding the intro in last, and have to work hard to make sure it doesn't sound like a bolt on. When I hadn't used to pay attention to intro length, the used to come first and the song grew from there, still does sometimes.
Usually a chord sequence that's coming up in the song works well. Either the first part of the first verse or end of the chorus work well. If you're going for commercial sounding stuff, it's good to try and get some kind of a hook in there from the off, again, chorus material can be a good place to look if you're adding the intro in later.
I've got one old song where I decided to scrap the intro altogether and just go straight into v1. Interestingly it's my least favourite song of mine.
Yodasdad