In order to develop your interpersonal skills, you must enhance your intrapersonal communication. If your thoughts are clear and the mind and tongue coordinate well with each other, you will be a better speaker.
You should always spend some quality time with your own self every day. It will give you time to reflect on your personality and work on your flaws.
Listen to others
You will make life a whole lot easier for yourself and those around you by exercising tolerance and listening to people. Everyone has opinions he/she is entitled to hold. Just like you want others to respect your beliefs, you should do the same in return.
Work on shortcomings
Analyse your relations with people and try to gauge the reasons behind ineffective communication. If you find it hard to get through to a certain person, look into the possible reasons and work on rectifying any shortcomings.
Different people have different ways of both expressing themselves and sharing things - try to adapt.
Improve your language & speaking skills
Language is among the most important things in communication. You should weight and select your words very carefully. Also, be mindful of your tone and communication channel used.
If you are well dressed and your personality is nicely groomed, it is going to have a massive impact on your interpersonal skills. An attractive person finds it a lot easier to convince other people about his/her opinion as compared to those who do not care too much about their appearance.