I don't use them. Unit tests should be sufficient to test your code. Furthermore, since they're disabled by default, they're normally completely ignored anyways. Then they just tend to clutter up your code with useless assertions that could be better expressed as comments.
If you really need this, though, some libraries have assertion static methods you can call that will not be skipped - these are also a lot more readable, because the assert keyword is uncommon and immediately may cause a "wtf" moment, but the Assert.x methods are just methods that can be traced through. The Spring framework, in particular, uses an assertion library.
