Hi skeye!
I take lithium and have been on it off and on for almost a year. I say off and on as it took a little bit of time and playing around with the dosages to get it right. I have built up to 600 mg 2xday. I too am on lamictal - 150 2xday. Several good indicators that you are on too much lithium is shaking hands or shakiness in other limbs, clumsiness walking, lethargy and the feeling like you just can't get up to do anything. With me they overshot the right lithium level and I became toxic with the above symptoms, but the symptoms are obvious and you just make sure you call about them right away. They lowered my dose and in a couple of days the symptoms were gone. Do drink lots of water. I find I am more thirsty, esp. in the summer, but it is manageable.
It takes a while for the lithium to build up in your system, as it does with lamictal, so don't expect sudden improvements. Due to my personal situation, I have found that the lithium and lamictal have stabilized me - meaning I still feel depressed but I think the depression that is there is situational at this point. I have been told be everyone close to me that I am significantly better.
Hope this is helpful and I would be happy to try to answer any other questions you have.
soleilsun