The Belastingdienst (IR in England; IRS in America) and the UVW (Dutch department for social security) are big and mighty. You don't want to lose energy by going into battle with them. Keeping them satisfied is for the most part a matter of keeping yours things organized. Make sure your administration is up to date and know what tax assessment you await.

VAR for small businesses
Most employers (or employers of employers) require a VAR (or "Verklaring ArbeidsRelatie") from independent IT contractors. You obtain this from the Dutch Belastingdienst. In the application form you describe some essential details of your relationship with your customers. Futhermore, you specify how you want the customer to handle your tax liability. It is very likely that you want to be treated as having your own company, rather than the taxes being withhold by your customer. The VAR relieves the customer from this burden.

Flexlaw
Not everyone who claims to run a company is actually treated as such by the law. The Flexlaw (flexible worker law) poses some restrictions on the relationship employee-employer. The most important one being the number of times a contract has been renewed. If you work for an employer for more than 3 years, or your contract has been extended 4 times or more, your contract will automaticly be extended indefinitely. When you are treated as an employee, you'll loose certain benefits of being a company.

Who pays and who gains?
Who will pay income tax and social security contributions? How often and when?
Who will pay VAT? How often and when?
Who will pay company tax?
Who will get tax relieve?