Someone else posted this feeding ratio, but it's proven to work VERY well for all my starters.
I keep my starters in the refrigerator and I feed them once a week. I just take out the jar and let the starter come to room temp over the course of couple hours. I know the weight of my containers when they're empty, so when I weigh them, it's easy to determine the total weight of the starter in the jar.
So determine how much starter you want to keep. I generally keep 20-30 grams. So I discard everything but 20 grams and then I add 2 parts new flour (40 grams) and 2 parts water (another 40 grams). This 1/2/2 ratio has always produced nearly a tripling in volume over the course of 4-5 hours.
Before this, I was discarding about half of the starter and replacing that half (a 1/ .5/ .5 ratio) and my starters never seemed to be active enough.