Why didn't Lelouch use the "obey me" order on everyone? In the final episodes of Code Geass R2, Lelouch gave orders like "be my slave" or "obey me". This way, unlike when using the Geass, he can give them orders an unlimited number of times.


(Code Geass Wikia)

So why didn't he use these orders on everyone from the beginning? This way he could've given them orders whenever he wanted!
At the very beginning, right after he got his powers, Lelouch may not have been sure that it was possible to give such powerful orders. This is why, for example, Lelouch ordered a female student from the academy to engrave a cross-mark on a wall every day forever: to determine how long a geass would last. Presumably, he was performing similar experiments that weren't shown to the viewer. 

In some cases, Lelouch could be morally conflicted about geassing people. In the last episode of R2, we see him wavering on whether or not to use a geass to command Nunnally to give him the FLEIJA control device. It's possible that he felt the same way about using his geass on other people as well. (Then again, that might just be a special thing for Nunnally.)

Recall now that people are able to reject a geass (at least temporarily) if it is in strong conflict with their morals. Guilford, for example, would probably have found the idea of obeying Zero to be utterly repugnant, and may have resisted the geass to a degree, which could have thrown a wrench in Lelouch's plans. This could be why Lelouch instead geassed Guilford to view him as if he were Cornelia - to circumvent that issue.

There are probably a few other reasons, too, which hopefully other people will be able to come up with in their answers.