In short, you can't protect against having your stake diluted.
To keep it simple, assume that the company is always 100% allocated to the current shareholders after a given round (it often isn't, there are some shares reserved for certain things like new hires, but for the purposes of discussion this explanation will work).
Let's say a company has 10M million shares, allocated to employees and investors.
So, you have 10% (1M Shares), the founder has 50% (5M) and the other 40% (4M) is held by the Round A investors.
Now, it's time for Round B. All the current stock is allocated, so there are no shares to sell. This is solved by issuing more shares for the new investors to buy. The sum total of all the shares always has to be 100%. So if we issue another 10M shares, the company now has 20M shares of stock. You still have 1 Million shares, but it's 1 Million of 20 Million, instead of 1 Million of 10 Million. 5% instead of 10%.
This is a very simple explanation. It can be further complicated by the fact that you may have multiple classes of stock, or different terms for different share holders. It is common to have a provision such that if the Round A investors re-invest in Round B, then their shares do not get diluted, or get diluted less than your common shares.
In essence, almost any scenario for dilution is fair game, and precedence will always go to investors over option grantees.