First of all, upload to the arxiv. Then you can write to the experts you know, and inform them that you uploaded your new paper. Then wait a little to give them opportunity to send you comments. If there is a possibility, give seminar/conference talks, but it is not necessary to do this before you send it
to a journal.  

Edit. Posting the paper in the arxiv is really the most important step. See the comment of Timothy Chow for many reasons of this. From the point of view of 
dissemination of the result, it is more important than publishing it in a journal. Some people who do not care about grants and promotions even do not bother to send their results to journals.