You don't. All new share issuance's dilute all shareholders. You can have different classes like A shares and B shares, convertibles and options structured in a way that will maintain your position. But then no investor in his right mind would be willing to invest.
Just remember a small piece of a big pie is worth more than a big piece of a very small pie or no pie at all. Don't get wrapped around axle on trying to keep a majority in later rounds of financing.
Its a math problem, if you issue 500k more shares you figure out what amount of the shares you would have to issue to yourself to maintain your 50% control. In that case if you gave yourself 250k of the 500k new shares you'd have 750k of 1.5 million shares, easy enough, you could not, however, issue new shares or bring on another equity investor without diluting the share that someone had in the company.