Just recent I had that problem too.Its most probably that your charging system is spoilt.In this situation, there's no short cut other than taking for repair. In case you don't mend it faster, then the condition will worsen and this might affect your battery life.
In case the charging system is alright, then try charging it when the phone is off. If you don't feel like charging it while it off, then you can switch it on but put on the airplane mode.I have tried this several times and it has worked perfectly.
Finally, when charging, use a direct connection from the wall sockets. Sometimes charging a phone with a laptop charges it slowly.