Cross-post exercise question

I usually hang out on the DS board, but was referred here with this question...I would appreciate your expertise. I started exercising again at the first of the year, and for the past 6 weeks I have been getting my five days a week in religiously (Good for me!). I do 30 minutes on the treadmill 3 days a week, the other two days I do ten minutes on the treadmill, then strength-building, then stretches. For the strength-building I am using an I-Max machine (knock-off of  the BowFlex). My goals are not to build giant muscles but to tone and preserve muscle and strengthen my bones (osteoporosis prevention). I started out with very low weights, for example, ten pounds on the bicep curls and 180 pounds on the leg presses. I have heard that lots of reps with relatively low weights are best for my goals. And I have built up to 3 sets of 15 reps on each exercise. But now that I have built up to 15 reps, I probably do need to increase the weights some.  So here is my question..... How much? 10%? 25%? More? Less? Is there a point at which I want to stop adding weight and just continue at that level?

I can't really answer your question about the amount of weight/resistance to add, but I have a comment about building muscle.  I've been reading a lot of women's fitness magazines, and they say that most women don't need to worry about building large muscles, even with higher weights.  In order to build big muscles, you need a higher level of testosterone than most woman have naturally.   From what I've read, you should lift the amount of weight that allows you to do 8-10 reps before you're maxed out for that set.  When you get to the point that you can do 15 per set, then it's time to slightly raise the weight.   I am *no* authority on this, though, that's just what I've been picking up.  Tami