Damn, studying for Highers, that takes me back.
My advice would be this - pace yourself. Don't try to cram at any stage. (it works for some people I guess, but you'd essentially be clutching at straws. Not a good strategy.) Make a plan of sorts of what you want to go over. Plug the gaps in your knowledge first, then move on to refining what you're more sure of.
If you run into anything you don't understand, don't freak out - it happens to everyone at this stage. Really. Just be calm, look over the material, and if you're still not getting it ask someone for help. Don't be afraid to approach your teacher - the majority of them will be more than happy to help. Getting you through these exams is their job (and the more people get good grades, the better they'll look, after all).
When you're done revising a particular topic, I think it's really helpful to get someone to ask you questions about it. Best if it's someone who also knows the subject (classmates, essentially), but really, anyone can help with this. If there's anything you struggle to answer, you'll know it still needs a little work.
Oh, and past papers are a must. The SQA ones are actually surprisingly helpful. (Or they were when I was sitting their exams, anyway.)
Hope that's of some use. Good luck dude, hope you get the grades you need for uni/college/apprenticeship/whatever it is you want to do!