Doubts about whether some drops of urine have fallen on clothes

It happens that after I have cleaned myself (after passing water) and done Wudu, I feel some drops of urine come out, which makes me repeat my wudoo, sometimes twice. After the urine has come out, I wait for five minutes until I am sure that it has stopped, but after I finish Wudu, I feel that a few small drops of urine have come out of me. I am trying to solve this problem so that I will have no doubts at all. If it happens that a few drops of urine get onto my clothes, do I have to wash it? If this happens, does it invalidate my prayer? Sometimes I start washing my trousers because I am not sure whether there are any drops of urine on them.