I'm pretty sure that theres a jet(1); command that keeps jet on unless you say jet(0).
Same with crouch... I think....
To answer your question; here are the commands and how to use them:
Jet(1);
That will keep your jet on untill you press and release the jet button. (in most cases, right mouse button.)
It can also be disabled by using Jet(0); or Jet(false);
Crouch(1);
That will keep you in the crouched position untill you press and release the crouch button.
It can also be disabled with Crouch(0); or Crouch(false);