Submissive? Slave? Sir? Controlled Sir? Hmmmm?

I have been trying to field the wonderful world of internet porn trying to find information on understanding the deeper levels of submission in a sense of comparing the difference between submissive and slave. I haven't been too successful on the information gathering so please forgive me if this question is sounds a bit naive, but i have a questions on when that level of submission goes deep enough to the point that the need to fallow goes beyond ones personal limits, when the need to please goes beyond a sense of self preservation and voice of reason, when the need to please out shines the gift of choice?
My first question?
What advice would you give to a sub/slave when seeking out a Sir/Master?

As i have already stated? i haven't been too successful on finding information so with my limited understanding i now am a little confused between the two, submissive and slave. Please don't take offense to my wording and correct me if i am wrong here, but aside from the more obvious 24/7 versus negotiated times, is there more that differentiate between the two? And if there is a line, is there a gray area where one can be both?

Also, in passing i have been told that in these deeper levels of submission that it is important to find a Sir/Master that not only can control the sub/slave but also can control Him/Herself to best avoid the temptation of unnecessary cruelty. Now, i understand the 'temptation of unnecessary cruelty', I'm not so sure about understanding the 'control Him/Herself' part. How does one know how to judge how much control one has during introductions and negotiations? Or is it pretty much a crap shoot and hope for the best?

For simplicity:
"she" will be the bottom/sub/slave
"he" will be the top/dom/master

Seems to me (from my own contemplation) that first she has to be able to trust herself. In the cold light of day, she must decide her limits. She must decide what will be for her self-nurturing, and what would cross over into self-abuse. She has to make a pact with herself that she will obey her own limits even in the heat of passion.
Perhaps she has a fear that when when things get hot, she won't respect her own limits. She must decide that she will not renegotiate hard limits when the pain is exquisite and she desperately wants more.
More intensity,
to please Him more,
to give away more,
to be used for the pleasure of Master MORE.

There must be a point where the depth of the experience is enough. The experience is good.
This reminds me of alcohol. I have been drunk. I have thought I wanted a deeper experience. I wanted more. But more alcohol crosses over into getting sick. (Been there. Done that). So with alcohol you have to realize that "this" is good. "This" is enough. I have reached a good place and it is enough.

I think the same must be true for BDSM. You have to be able to reconcile that "this" is good, "this" is great. I have reached a good place and it is enough.

But BDSM has a separate issue: Him.
He has to be trusted to obey her limits. She puts her safety into His hands. She can safeword only if she can still safeword. If he can turn her brain off then he alone is responsible for her safety and her limits.
In the heat of passion he must maintain enough control to obey her limits and his own limits as well.
With a new play partner, maybe he gets only ONE chance. If he does not respect a safeword or does not respect a limit. He should be gone.
Hard limits can be renegotiated but MUST be negotiated only in the cold light of day. If he wants to change a hard limit in the heat of passion he cannot be trusted. He should be gone.