A standard approach consists of using energy estimates techniques. The latter can be used to prove dissipation for a broad class of ODEs (and PDEs).
The example of the Lorenz model (63) is standard and gives already some insights on how energy estimates operate. For this model, a proof of the boundedness of the trajectories can be found for instance in the book of Roger Temam (Chapter I, sec. 2.3) "Infinite-Dimensional Dynamical Systems in Mechanics and Physics."