Note: This thread contains matters related to men only. Nowadays a lot of muslim men are not clear of these issues which breaks wudhu.
In the Name of Allah, Most Compassionate, Most Merciful,
In order for one to initially be considered as having a chronic excuse (`udhr), one’s excuse (that renders one unable to retain wudu) must be present an entire prayer time, such that one cannot perform a minimal wudu and pray a quick prayer without the excuse taking place.
Secondly, in order for one to remain legally excused (ma`dhur), one’s excuse has to occur at least once within the obligatory prayer time.
If these conditions exist, then one will not have to make up one’s prayer. Otherwise, one will have to make up that prayer, and one is sinful for making the time go out without repeating it.
“It is necessary to take all reasonable means to stop that which makes one unable to retain ones wudu. So, for example, if it is possible to pray sitting without the excuse (such as releasing air) occurring, one must pray sitting.”
Suggestion: It may be that when one uses the lavatory to relieve oneself that one is too quick to wash one’s front private part before all drops of urine have ceased to exit.
"After relieving oneself, it is obligatory that one free oneself of any remaining traces of urine such that nothing will come out after one s wudu.
One should wait a little after urinating; free oneself of traces of urine by coughing, swaying or the like. If one tugs , the scholars say it should only be done very gently a couple of times, as this can harm one and also leads to long term waswasa. When reasonably sure that no more urine is coming out one should get up and:
- sprinkle water lightly on one s private parts and underwear, as the Prophet (Allah bless him & give him peace) instructed,
- and then walk around a little,
- and then do wudu."
Then, if you have any doubts about urine, you simply assume the wetness you think you feel is from the sprinkled water.
Source: http://qa.sunnipath.com/issue_view.asp?HD=1&ID=5116&CATE=239