How do you all decide when you need to raise (or, maybe, lower) the dose of antidepressants you are taking?

I am doing okay at 20mg of lexapro for the moment, but the doctor seems to think 30mg is in my future. But I don't know when to make that decision.

I've been on this dose for 3 weeks and have seen improvement, but it's so hard to know what to do.

thea