Getting the illusion of roundness?

[Punk-A-Cat]

I think my entire ability to do perspective is lacking.. I understand the theory, I can do /some/ of it - atmospheric stuff comes naturally as we studied that at school and had it drilled into us, so many landscape studies - but I'm still crap at landscapes, and at anything perspective requiring in general. I'm working through the perspective course on here (I bookmarked it) but even the most basic stuff isn't right. I hope mileage will help, I think part of it is I've always been messy, and having spent two years hating the hell out of technical drawing in high school I look at rulers with a full on loathing. :D