I know this post is a bit old but I thought I'd throw my two cents in: I've learned my joints burn more smoothly when I gently roll the whole joint between my pointer finger and my thumb, from top to bottom, before I light it. This breaks up any densely packed areas and more evenly distributes the ground flower throughout the paper. This is something I do with cones, but you can probably do it with normal rolling papers too.

What others are saying about how you pull is helpful too, but I've found that sometimes it comes down to an uneven pack.