When my hair was shorter, my stylist did a sort of modified undercut on it. It's similar to what Jules describes, but not as extreme (so not shaved at the nape, but shorter and thinned out). In addition to lightening the weight, it also prevented my hair from forming a triangle shape (because it's very straight and heavy, the top would get weighed down while the bottom would pouf out). Now that it's longer, she doesn't do it any more, because it prevents you from being able to put up your hair nicely. So I'd recommend this type of undercut for shorter (mine was around shoulder length) but not necessarily longer hair.
I do get headaches from ponytails (and even the twist I normally put it up in starts to ache by the end of the day) but it's so straight I don't get many tangles so I can't help you there.