Weinberg shows in his book on QFT that the effect of a rotation on a massive particle's spin is the same as applying exp(-i \theta J.n) on the state, regardless of particle's momentum. In other words just rotate the spinor in its own representation. This makes calculations very simple. But what about boosts? There is of course a general approach described in Weinberg's book using little group elements; but it is too difficult to apply it each time. Is there any genral theorem for boost like that Weinberg presented for rotations? And also what about massless particles?