It is possible to significantly improve your selling skills in a relatively short space of time without any more sales training and long as you are prepared to take charge of your own learning. Following these simple steps, you can become your own sales coach:
Prioritise

Which selling skills, specifically, do you want to develop? There are many areas of sales and in order to develop them you need to start paying attention to specific ones.

When you really get into it, and start getting into the detail, the list of potential selling skills is endless. However, you have to start somewhere and so pick a general area of sales that is likely to give you the biggest boost in results and start there. Here are six major skill areas, each with specific skills to be learnt:

Generating interest
Qualifying interest
Discovering needs and wants
Proposing your solution
Negotiating and handling objections
Closing the sale

When you start breaking each skill area down, there are a lot of soft skills like developing rapport, reading non-verbal communication, listening, asking clean questions. The point is that you need to get very specific about which skills you want to develop, prioritise them and start working on them one-by-one.

In my book The Sales Coach I include a number of diagnostic tests to help you assess your skill levels and pin-point which selling skills to focus on first. The main diagnostic test is in the free sample available on the book website.
Seek out excellence

Start looking out for people who have the level of skill you want to develop and if possible, watch them at work. If you have colleagues with superior skills then ask if you could spend sometime watching them in action. When you are a buyer in your personal life, look for good and bad examples of selling skill.
Review and refine

An important thing to remember in developing a skill is that the road to mastery is a process of continual refinement. You need to review your performance, give yourself credit for what you did well and decide on a couple of areas to improve next time.
Get external feedback

Continually reviewing and refining your own performance will help you make significant improvements but there are likely to be some blind spots that are outside of your awareness. So, if you are really serious then you would get a manager, coach, or colleague to observe you in action and give you feedback. Make sure you brief them in advance as to what kind of feedback you want and how you want it to be delivered.
Keep at it

Remember that Rome was not built in a day and if you seek to get 1% better each day then over the course of 3 months that adds up to over 65%. Set yourself learning goals and targets and stay committed despite the inevitable set backs. If you struggle with self-discipline then get a manager or coach onboard to support you. The good thing about sales is that improved skills should equate to increased income which means that any expenditure of coaching is effectively an investment to increase future earnings.