Let the other person finish considering an offer before improving it out of fear.

Silence during a decision is uncomfortable, and the reflex is to fill it by sweetening the terms. Very often the person was simply thinking, or was going to say yes, and the improvement bought nothing except a lower starting point and a hint that you were anxious. You end up negotiating against yourself while nobody is opposing you.

After you make an offer, stop talking and let the pause be theirs. If you need to fill the wait, ask when they expect to know rather than adding something they never asked for.