I'd recommend just reading a ton of books. There's a formula to sales that is well documented, you just have to adapt the pieces that work with your personality. The ones I'd recommend reading are:
* SPIN Selling
* Never Split the Difference
* Getting to Yes
* To Sell is Human
* Read any free information online about SPIN/NEAT/Sandler
I read all of those cover-to-cover right before I started my consulting business and it made a marked impact on my ability to sell. Like anything, it's a combination of study and practice. Read a book, figure out the nuggets that are important, then practice those tactics. Learn, rinse, repeat.