Question about vitakka and vicara. How to reinforce upper jhanas (without vitakka and vicara)

Sometimes i am keeping some mental object in mind, free from anger or greed, and for a moment i feel piti and sukha over my entire body. Sometimes this piti and sukha keep with me for a long time. But even in first jhana i keep my mental object in mind, like blue kasina (whondering world all blue) or parts of body. Sometimes the piti and sukha leaves. I read that after the first jhana we need to abandon the vitakka and vicara. But how? I need to keep my focus in some mental object or just let the things go at their own pace, in my mind. How to i reinforce the upper jhanas if i need to abandon the mental objects? Ps: sometimes i feel piti and sukha, when concentrate into the kammathana, both in public and in my cave (bedroom).